diff --git a/packages/cli/src/commands/extensions/examples/themes-example/README.md b/packages/cli/src/commands/extensions/examples/themes-example/README.md
new file mode 100644
index 0000000000..b8eb87229c
--- /dev/null
+++ b/packages/cli/src/commands/extensions/examples/themes-example/README.md
@@ -0,0 +1,28 @@
+# Themes Example
+
+This is an example of a Gemini CLI extension that adds a custom theme.
+
+## How to use
+
+1.  Link this extension:
+
+    ```bash
+    gemini extensions link packages/cli/src/commands/extensions/examples/themes-example
+    ```
+
+2.  Set the theme in your settings file (`~/.gemini/config.yaml`):
+
+    ```yaml
+    ui:
+      theme: 'shades-of-green-theme (themes-example)'
+    ```
+
+    Alternatively, you can set it through the UI by running `gemini` and then
+    typing `/theme` and pressing Enter.
+
+3.  **Observe the Changes:**
+
+    After setting the theme, you should see the changes reflected in the Gemini
+    CLI's UI. The background will be a dark green, the primary text a lighter
+    green, and various other UI elements will display different shades of green,
+    as defined in this extension's `gemini-extension.json` file.
diff --git a/packages/cli/src/commands/extensions/examples/themes-example/gemini-extension.json b/packages/cli/src/commands/extensions/examples/themes-example/gemini-extension.json
new file mode 100644
index 0000000000..47a26c5105
--- /dev/null
+++ b/packages/cli/src/commands/extensions/examples/themes-example/gemini-extension.json
@@ -0,0 +1,29 @@
+{
+  "name": "themes-example",
+  "version": "1.0.0",
+  "themes": [
+    {
+      "name": "shades-of-green-theme",
+      "type": "custom",
+      "background": {
+        "primary": "#1a362a"
+      },
+      "text": {
+        "primary": "#a6e3a1",
+        "secondary": "#6e8e7a",
+        "link": "#89e689"
+      },
+      "status": {
+        "success": "#76c076",
+        "warning": "#d9e689",
+        "error": "#b34e4e"
+      },
+      "border": {
+        "default": "#4a6c5a"
+      },
+      "ui": {
+        "comment": "#6e8e7a"
+      }
+    }
+  ]
+}
diff --git a/packages/cli/src/config/extension-manager-themes.spec.ts b/packages/cli/src/config/extension-manager-themes.spec.ts
new file mode 100644
index 0000000000..90b0363e92
--- /dev/null
+++ b/packages/cli/src/config/extension-manager-themes.spec.ts
@@ -0,0 +1,226 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import * as fs from 'node:fs';
+import * as path from 'node:path';
+import {
+  beforeAll,
+  afterAll,
+  beforeEach,
+  describe,
+  expect,
+  it,
+  vi,
+  afterEach,
+} from 'vitest';
+import { createExtension } from '../test-utils/createExtension.js';
+import { ExtensionManager } from './extension-manager.js';
+import { themeManager, DEFAULT_THEME } from '../ui/themes/theme-manager.js';
+import { GEMINI_DIR, type Config } from '@google/gemini-cli-core';
+import { createTestMergedSettings, SettingScope } from './settings.js';
+
+describe('ExtensionManager theme loading', () => {
+  let extensionManager: ExtensionManager;
+  let userExtensionsDir: string;
+  let tempHomeDir: string;
+
+  beforeAll(async () => {
+    tempHomeDir = await fs.promises.mkdtemp(
+      path.join(fs.realpathSync('/tmp'), 'gemini-cli-test-'),
+    );
+  });
+
+  afterAll(async () => {
+    if (tempHomeDir) {
+      await fs.promises.rm(tempHomeDir, { recursive: true, force: true });
+    }
+  });
+
+  beforeEach(() => {
+    process.env['GEMINI_CLI_HOME'] = tempHomeDir;
+    userExtensionsDir = path.join(tempHomeDir, GEMINI_DIR, 'extensions');
+    // Ensure userExtensionsDir is clean for each test
+    fs.rmSync(userExtensionsDir, { recursive: true, force: true });
+    fs.mkdirSync(userExtensionsDir, { recursive: true });
+
+    extensionManager = new ExtensionManager({
+      settings: createTestMergedSettings({
+        experimental: { extensionConfig: true },
+        security: { blockGitExtensions: false },
+        admin: { extensions: { enabled: true }, mcp: { enabled: true } },
+        tools: { enableHooks: true },
+      }),
+      requestConsent: async () => true,
+      requestSetting: async () => '',
+      workspaceDir: tempHomeDir,
+      enabledExtensionOverrides: [],
+    });
+    vi.clearAllMocks();
+    themeManager.clearExtensionThemes();
+    themeManager.loadCustomThemes({});
+    themeManager.setActiveTheme(DEFAULT_THEME.name);
+  });
+
+  afterEach(() => {
+    delete process.env['GEMINI_CLI_HOME'];
+  });
+
+  it('should register themes from an extension when started', async () => {
+    const registerSpy = vi.spyOn(themeManager, 'registerExtensionThemes');
+    createExtension({
+      extensionsDir: userExtensionsDir,
+      name: 'my-theme-extension',
+      themes: [
+        {
+          name: 'My-Awesome-Theme',
+          type: 'custom',
+          text: {
+            primary: '#FF00FF',
+          },
+        },
+      ],
+    });
+
+    await extensionManager.loadExtensions();
+
+    const mockConfig = {
+      getEnableExtensionReloading: () => false,
+      getMcpClientManager: () => ({
+        startExtension: vi.fn().mockResolvedValue(undefined),
+      }),
+      getGeminiClient: () => ({
+        isInitialized: () => false,
+        updateSystemInstruction: vi.fn(),
+        setTools: vi.fn(),
+      }),
+      getHookSystem: () => undefined,
+      getWorkingDir: () => tempHomeDir,
+      shouldLoadMemoryFromIncludeDirectories: () => false,
+      getDebugMode: () => false,
+      getFileExclusions: () => ({
+        isIgnored: () => false,
+      }),
+      getGeminiMdFilePaths: () => [],
+      getMcpServers: () => ({}),
+      getAllowedMcpServers: () => [],
+      getSanitizationConfig: () => ({
+        allowedEnvironmentVariables: [],
+        blockedEnvironmentVariables: [],
+        enableEnvironmentVariableRedaction: false,
+      }),
+      getShellExecutionConfig: () => ({
+        terminalWidth: 80,
+        terminalHeight: 24,
+        showColor: false,
+        pager: 'cat',
+        sanitizationConfig: {
+          allowedEnvironmentVariables: [],
+          blockedEnvironmentVariables: [],
+          enableEnvironmentVariableRedaction: false,
+        },
+      }),
+      getToolRegistry: () => ({
+        getTools: () => [],
+      }),
+      getProxy: () => undefined,
+      getFileService: () => ({
+        findFiles: async () => [],
+      }),
+      getExtensionLoader: () => ({
+        getExtensions: () => [],
+      }),
+      isTrustedFolder: () => true,
+      getImportFormat: () => 'tree',
+    } as unknown as Config;
+
+    await extensionManager.start(mockConfig);
+
+    expect(registerSpy).toHaveBeenCalledWith('my-theme-extension', [
+      {
+        name: 'My-Awesome-Theme',
+        type: 'custom',
+        text: {
+          primary: '#FF00FF',
+        },
+      },
+    ]);
+  });
+
+  it('should revert to default theme when extension is stopped', async () => {
+    const extensionName = 'my-theme-extension';
+    const themeName = 'My-Awesome-Theme';
+    const namespacedThemeName = `${themeName} (${extensionName})`;
+
+    createExtension({
+      extensionsDir: userExtensionsDir,
+      name: extensionName,
+      themes: [
+        {
+          name: themeName,
+          type: 'custom',
+          text: {
+            primary: '#FF00FF',
+          },
+        },
+      ],
+    });
+
+    await extensionManager.loadExtensions();
+
+    const mockConfig = {
+      getWorkingDir: () => tempHomeDir,
+      shouldLoadMemoryFromIncludeDirectories: () => false,
+      getWorkspaceContext: () => ({
+        getDirectories: () => [],
+      }),
+      getDebugMode: () => false,
+      getFileService: () => ({
+        findFiles: async () => [],
+      }),
+      getExtensionLoader: () => ({
+        getExtensions: () => [],
+      }),
+      isTrustedFolder: () => true,
+      getImportFormat: () => 'tree',
+      getFileFilteringOptions: () => ({
+        respectGitIgnore: true,
+        respectGeminiIgnore: true,
+      }),
+      getDiscoveryMaxDirs: () => 200,
+      getMcpClientManager: () => ({
+        getMcpInstructions: () => '',
+        startExtension: vi.fn().mockResolvedValue(undefined),
+        stopExtension: vi.fn().mockResolvedValue(undefined),
+      }),
+      setUserMemory: vi.fn(),
+      setGeminiMdFileCount: vi.fn(),
+      setGeminiMdFilePaths: vi.fn(),
+      getEnableExtensionReloading: () => true,
+      getGeminiClient: () => ({
+        isInitialized: () => false,
+        updateSystemInstruction: vi.fn(),
+        setTools: vi.fn(),
+      }),
+      getHookSystem: () => undefined,
+      getProxy: () => undefined,
+      getAgentRegistry: () => ({
+        reload: vi.fn().mockResolvedValue(undefined),
+      }),
+    } as unknown as Config;
+
+    await extensionManager.start(mockConfig);
+
+    // Set the active theme to the one from the extension
+    themeManager.setActiveTheme(namespacedThemeName);
+    expect(themeManager.getActiveTheme().name).toBe(namespacedThemeName);
+
+    // Stop the extension
+    await extensionManager.disableExtension(extensionName, SettingScope.User);
+
+    // Check that the active theme has reverted to the default
+    expect(themeManager.getActiveTheme().name).toBe(DEFAULT_THEME.name);
+  });
+});
diff --git a/packages/cli/src/config/extension-manager.ts b/packages/cli/src/config/extension-manager.ts
index 7e05df554a..a7add924be 100644
--- a/packages/cli/src/config/extension-manager.ts
+++ b/packages/cli/src/config/extension-manager.ts
@@ -69,6 +69,7 @@ import {
   ExtensionSettingScope,
 } from './extensions/extensionSettings.js';
 import type { EventEmitter } from 'node:stream';
+import { themeManager } from '../ui/themes/theme-manager.js';
 
 interface ExtensionManagerParams {
   enabledExtensionOverrides?: string[];
@@ -468,6 +469,20 @@ Would you like to attempt to install via "git clone" instead?`,
     );
   }
 
+  protected override async startExtension(extension: GeminiCLIExtension) {
+    await super.startExtension(extension);
+    if (extension.themes) {
+      themeManager.registerExtensionThemes(extension.name, extension.themes);
+    }
+  }
+
+  protected override async stopExtension(extension: GeminiCLIExtension) {
+    await super.stopExtension(extension);
+    if (extension.themes) {
+      themeManager.unregisterExtensionThemes(extension.name, extension.themes);
+    }
+  }
+
   /**
    * Loads all installed extensions, should only be called once.
    */
@@ -698,6 +713,7 @@ Would you like to attempt to install via "git clone" instead?`,
         resolvedSettings,
         skills,
         agents: agentLoadResult.agents,
+        themes: config.themes,
       };
       this.loadedExtensions = [...this.loadedExtensions, extension];
 
diff --git a/packages/cli/src/config/extension.ts b/packages/cli/src/config/extension.ts
index bafaba59a8..b6256fc83b 100644
--- a/packages/cli/src/config/extension.ts
+++ b/packages/cli/src/config/extension.ts
@@ -7,6 +7,7 @@
 import type {
   MCPServerConfig,
   ExtensionInstallMetadata,
+  CustomTheme,
 } from '@google/gemini-cli-core';
 import * as fs from 'node:fs';
 import * as path from 'node:path';
@@ -27,6 +28,11 @@ export interface ExtensionConfig {
   contextFileName?: string | string[];
   excludeTools?: string[];
   settings?: ExtensionSetting[];
+  /**
+   * Custom themes contributed by this extension.
+   * These themes will be registered when the extension is activated.
+   */
+  themes?: CustomTheme[];
 }
 
 export interface ExtensionUpdateInfo {
diff --git a/packages/cli/src/config/settingsSchema.ts b/packages/cli/src/config/settingsSchema.ts
index ff8ea13c55..9cfde9fab6 100644
--- a/packages/cli/src/config/settingsSchema.ts
+++ b/packages/cli/src/config/settingsSchema.ts
@@ -9,19 +9,17 @@
 // to regenerate the settings reference in `docs/get-started/configuration.md`.
 // --------------------------------------------------------------------------
 
-import type {
-  MCPServerConfig,
-  BugCommandSettings,
-  TelemetrySettings,
-  AuthType,
-  AgentOverride,
-} from '@google/gemini-cli-core';
 import {
   DEFAULT_TRUNCATE_TOOL_OUTPUT_LINES,
   DEFAULT_TRUNCATE_TOOL_OUTPUT_THRESHOLD,
   DEFAULT_MODEL_CONFIGS,
+  type MCPServerConfig,
+  type BugCommandSettings,
+  type TelemetrySettings,
+  type AuthType,
+  type AgentOverride,
+  type CustomTheme,
 } from '@google/gemini-cli-core';
-import type { CustomTheme } from '../ui/themes/theme.js';
 import type { SessionRetentionSettings } from './settings.js';
 import { DEFAULT_MIN_RETENTION } from '../utils/sessionCleanup.js';
 
diff --git a/packages/cli/src/test-utils/createExtension.ts b/packages/cli/src/test-utils/createExtension.ts
index f7ad425f06..56d02e7053 100644
--- a/packages/cli/src/test-utils/createExtension.ts
+++ b/packages/cli/src/test-utils/createExtension.ts
@@ -9,12 +9,13 @@ import * as path from 'node:path';
 import {
   type MCPServerConfig,
   type ExtensionInstallMetadata,
+  type ExtensionSetting,
+  type CustomTheme,
 } from '@google/gemini-cli-core';
 import {
   EXTENSIONS_CONFIG_FILENAME,
   INSTALL_METADATA_FILENAME,
 } from '../config/extensions/variables.js';
-import type { ExtensionSetting } from '../config/extensions/extensionSettings.js';
 
 export function createExtension({
   extensionsDir = 'extensions-dir',
@@ -25,12 +26,20 @@ export function createExtension({
   mcpServers = {} as Record<string, MCPServerConfig>,
   installMetadata = undefined as ExtensionInstallMetadata | undefined,
   settings = undefined as ExtensionSetting[] | undefined,
+  themes = undefined as CustomTheme[] | undefined,
 } = {}): string {
   const extDir = path.join(extensionsDir, name);
   fs.mkdirSync(extDir, { recursive: true });
   fs.writeFileSync(
     path.join(extDir, EXTENSIONS_CONFIG_FILENAME),
-    JSON.stringify({ name, version, contextFileName, mcpServers, settings }),
+    JSON.stringify({
+      name,
+      version,
+      contextFileName,
+      mcpServers,
+      settings,
+      themes,
+    }),
   );
 
   if (addContextFile) {
diff --git a/packages/cli/src/ui/components/ThemeDialog.tsx b/packages/cli/src/ui/components/ThemeDialog.tsx
index 00298d49d3..f04ae5172a 100644
--- a/packages/cli/src/ui/components/ThemeDialog.tsx
+++ b/packages/cli/src/ui/components/ThemeDialog.tsx
@@ -109,15 +109,6 @@ export function ThemeDialog({
     },
   );
 
-  // Generate theme items filtered by selected scope
-  const customThemes =
-    selectedScope === SettingScope.User
-      ? settings.user.settings.ui?.customThemes || {}
-      : settings.merged.ui.customThemes;
-  const builtInThemes = themeManager
-    .getAvailableThemes()
-    .filter((theme) => theme.type !== 'custom');
-  const customThemeNames = Object.keys(customThemes);
   const capitalize = (s: string) => s.charAt(0).toUpperCase() + s.slice(1);
 
   const terminalThemeType = getThemeTypeFromBackgroundColor(
@@ -125,8 +116,9 @@ export function ThemeDialog({
   );
 
   // Generate theme items
-  const themeItems = [
-    ...builtInThemes.map((theme) => {
+  const themeItems = themeManager
+    .getAvailableThemes()
+    .map((theme) => {
       const fullTheme = themeManager.getTheme(theme.name);
       const themeBackground = fullTheme
         ? resolveColor(fullTheme.colors.Background)
@@ -140,28 +132,14 @@ export function ThemeDialog({
         terminalBackgroundColor,
         terminalThemeType,
       );
-    }),
-    ...customThemeNames.map((name) => {
-      const themeConfig = customThemes[name];
-      const bg = themeConfig.background?.primary ?? themeConfig.Background;
-      const themeBackground = bg ? resolveColor(bg) : undefined;
-
-      return generateThemeItem(
-        name,
-        'Custom',
-        'custom',
-        themeBackground,
-        terminalBackgroundColor,
-        terminalThemeType,
-      );
-    }),
-  ].sort((a, b) => {
-    // Show compatible themes first
-    if (a.isCompatible && !b.isCompatible) return -1;
-    if (!a.isCompatible && b.isCompatible) return 1;
-    // Then sort by name
-    return a.label.localeCompare(b.label);
-  });
+    })
+    .sort((a, b) => {
+      // Show compatible themes first
+      if (a.isCompatible && !b.isCompatible) return -1;
+      if (!a.isCompatible && b.isCompatible) return 1;
+      // Then sort by name
+      return a.label.localeCompare(b.label);
+    });
 
   // Find the index of the selected theme, but only if it exists in the list
   const initialThemeIndex = themeItems.findIndex(
@@ -314,9 +292,20 @@ export function ThemeDialog({
                 };
 
                 if (item.themeNameDisplay && item.themeTypeDisplay) {
+                  const match = item.themeNameDisplay.match(/^(.*) \((.*)\)$/);
+                  let themeNamePart: React.ReactNode = item.themeNameDisplay;
+                  if (match) {
+                    themeNamePart = (
+                      <>
+                        {match[1]}{' '}
+                        <Text color={theme.text.secondary}>({match[2]})</Text>
+                      </>
+                    );
+                  }
+
                   return (
                     <Text color={titleColor} wrap="truncate" key={item.key}>
-                      {item.themeNameDisplay}{' '}
+                      {themeNamePart}{' '}
                       <Text color={theme.text.secondary}>
                         {item.themeTypeDisplay}
                       </Text>
diff --git a/packages/cli/src/ui/themes/theme-manager.test.ts b/packages/cli/src/ui/themes/theme-manager.test.ts
index 02ef4ff633..e80c03c5e1 100644
--- a/packages/cli/src/ui/themes/theme-manager.test.ts
+++ b/packages/cli/src/ui/themes/theme-manager.test.ts
@@ -11,7 +11,7 @@ if (process.env['NO_COLOR'] !== undefined) {
 
 import { describe, it, expect, beforeEach, afterEach, vi } from 'vitest';
 import { themeManager, DEFAULT_THEME } from './theme-manager.js';
-import type { CustomTheme } from './theme.js';
+import type { CustomTheme } from '@google/gemini-cli-core';
 import * as fs from 'node:fs';
 import * as os from 'node:os';
 import type * as osActual from 'node:os';
@@ -188,4 +188,54 @@ describe('ThemeManager', () => {
       consoleWarnSpy.mockRestore();
     });
   });
+
+  describe('extension themes', () => {
+    it('should register and unregister themes from extensions with namespacing', () => {
+      const extTheme: CustomTheme = {
+        ...validCustomTheme,
+        name: 'ExtensionTheme',
+      };
+      const extensionName = 'test-extension';
+      const namespacedName = `ExtensionTheme (${extensionName})`;
+
+      themeManager.registerExtensionThemes(extensionName, [extTheme]);
+      expect(themeManager.getCustomThemeNames()).toContain(namespacedName);
+      expect(themeManager.isCustomTheme(namespacedName)).toBe(true);
+
+      themeManager.unregisterExtensionThemes(extensionName, [extTheme]);
+      expect(themeManager.getCustomThemeNames()).not.toContain(namespacedName);
+      expect(themeManager.isCustomTheme(namespacedName)).toBe(false);
+    });
+
+    it('should not allow extension themes to overwrite built-in themes even with prefixing', () => {
+      // availableThemes has 'Ayu'.
+      // We verify that it DOES prefix, so it won't collide even if extension name is similar.
+      themeManager.registerExtensionThemes('Ext', [
+        { ...validCustomTheme, name: 'Theme' },
+      ]);
+      expect(themeManager.getCustomThemeNames()).toContain('Theme (Ext)');
+    });
+
+    it('should allow extension themes and settings themes to coexist', () => {
+      const extTheme: CustomTheme = {
+        ...validCustomTheme,
+        name: 'ExtensionTheme',
+      };
+      const settingsTheme: CustomTheme = {
+        ...validCustomTheme,
+        name: 'SettingsTheme',
+      };
+
+      themeManager.registerExtensionThemes('Ext', [extTheme]);
+      themeManager.loadCustomThemes({ SettingsTheme: settingsTheme });
+
+      expect(themeManager.getCustomThemeNames()).toContain(
+        'ExtensionTheme (Ext)',
+      );
+      expect(themeManager.getCustomThemeNames()).toContain('SettingsTheme');
+
+      expect(themeManager.isCustomTheme('ExtensionTheme (Ext)')).toBe(true);
+      expect(themeManager.isCustomTheme('SettingsTheme')).toBe(true);
+    });
+  });
 });
diff --git a/packages/cli/src/ui/themes/theme-manager.ts b/packages/cli/src/ui/themes/theme-manager.ts
index ef67f7fc25..c44c5adb98 100644
--- a/packages/cli/src/ui/themes/theme-manager.ts
+++ b/packages/cli/src/ui/themes/theme-manager.ts
@@ -18,7 +18,8 @@ import { ShadesOfPurple } from './shades-of-purple.js';
 import { XCode } from './xcode.js';
 import * as fs from 'node:fs';
 import * as path from 'node:path';
-import type { Theme, ThemeType, CustomTheme } from './theme.js';
+import type { Theme, ThemeType } from './theme.js';
+import type { CustomTheme } from '@google/gemini-cli-core';
 import { createCustomTheme, validateCustomTheme } from './theme.js';
 import type { SemanticColors } from './semantic-tokens.js';
 import { ANSI } from './ansi.js';
@@ -38,7 +39,9 @@ export const DEFAULT_THEME: Theme = DefaultDark;
 class ThemeManager {
   private readonly availableThemes: Theme[];
   private activeTheme: Theme;
-  private customThemes: Map<string, Theme> = new Map();
+  private settingsThemes: Map<string, Theme> = new Map();
+  private extensionThemes: Map<string, Theme> = new Map();
+  private fileThemes: Map<string, Theme> = new Map();
 
   constructor() {
     this.availableThemes = [
@@ -65,7 +68,7 @@ class ThemeManager {
    * @param customThemesSettings Custom themes from settings.
    */
   loadCustomThemes(customThemesSettings?: Record<string, CustomTheme>): void {
-    this.customThemes.clear();
+    this.settingsThemes.clear();
 
     if (!customThemesSettings) {
       return;
@@ -88,7 +91,7 @@ class ThemeManager {
 
         try {
           const theme = createCustomTheme(themeWithDefaults);
-          this.customThemes.set(name, theme);
+          this.settingsThemes.set(name, theme);
         } catch (error) {
           debugLogger.warn(`Failed to load custom theme "${name}":`, error);
         }
@@ -96,16 +99,103 @@ class ThemeManager {
         debugLogger.warn(`Invalid custom theme "${name}": ${validation.error}`);
       }
     }
-    // If the current active theme is a custom theme, keep it if still valid
+    // If the current active theme is a settings theme, keep it if still valid
     if (
       this.activeTheme &&
       this.activeTheme.type === 'custom' &&
-      this.customThemes.has(this.activeTheme.name)
+      this.settingsThemes.has(this.activeTheme.name)
     ) {
-      this.activeTheme = this.customThemes.get(this.activeTheme.name)!;
+      this.activeTheme = this.settingsThemes.get(this.activeTheme.name)!;
     }
   }
 
+  /**
+   * Loads custom themes from extensions.
+   * @param extensionName The name of the extension providing the themes.
+   * @param customThemes Custom themes from extensions.
+   */
+  registerExtensionThemes(
+    extensionName: string,
+    customThemes?: CustomTheme[],
+  ): void {
+    if (!customThemes) {
+      return;
+    }
+
+    debugLogger.log(
+      `Registering extension themes for "${extensionName}":`,
+      customThemes,
+    );
+
+    for (const customThemeConfig of customThemes) {
+      const namespacedName = `${customThemeConfig.name} (${extensionName})`;
+
+      // Check for collisions with built-in themes (unlikely with prefix, but safe)
+      if (this.availableThemes.some((t) => t.name === namespacedName)) {
+        debugLogger.warn(
+          `Theme name collision: "${namespacedName}" is a built-in theme. Skipping.`,
+        );
+        continue;
+      }
+
+      const validation = validateCustomTheme(customThemeConfig);
+      if (validation.isValid) {
+        if (validation.warning) {
+          debugLogger.warn(`Theme "${namespacedName}": ${validation.warning}`);
+        }
+        const themeWithDefaults: CustomTheme = {
+          ...DEFAULT_THEME.colors,
+          ...customThemeConfig,
+          name: namespacedName,
+          type: 'custom',
+        };
+
+        try {
+          const theme = createCustomTheme(themeWithDefaults);
+          this.extensionThemes.set(namespacedName, theme);
+          debugLogger.log(`Registered theme: ${namespacedName}`);
+        } catch (error) {
+          debugLogger.warn(
+            `Failed to load custom theme "${namespacedName}":`,
+            error,
+          );
+        }
+      } else {
+        debugLogger.warn(
+          `Invalid custom theme "${namespacedName}": ${validation.error}`,
+        );
+      }
+    }
+  }
+
+  /**
+   * Unregisters custom themes from extensions.
+   * @param extensionName The name of the extension.
+   * @param customThemes Custom themes to unregister.
+   */
+  unregisterExtensionThemes(
+    extensionName: string,
+    customThemes?: CustomTheme[],
+  ): void {
+    if (!customThemes) {
+      return;
+    }
+
+    for (const theme of customThemes) {
+      const namespacedName = `${theme.name} (${extensionName})`;
+      this.extensionThemes.delete(namespacedName);
+      debugLogger.log(`Unregistered theme: ${namespacedName}`);
+    }
+  }
+
+  /**
+   * Clears all registered extension themes.
+   * This is primarily for testing purposes to reset state between tests.
+   */
+  clearExtensionThemes(): void {
+    this.extensionThemes.clear();
+  }
+
   /**
    * Sets the active theme.
    * @param themeName The name of the theme to set as active.
@@ -133,13 +223,23 @@ class ThemeManager {
       const isBuiltIn = this.availableThemes.some(
         (t) => t.name === this.activeTheme.name,
       );
-      const isCustom = [...this.customThemes.values()].includes(
-        this.activeTheme,
-      );
+      const isCustom =
+        [...this.settingsThemes.values()].includes(this.activeTheme) ||
+        [...this.extensionThemes.values()].includes(this.activeTheme) ||
+        [...this.fileThemes.values()].includes(this.activeTheme);
 
       if (isBuiltIn || isCustom) {
         return this.activeTheme;
       }
+
+      // If the theme object is no longer valid, try to find it again by name.
+      // This handles the case where extensions are reloaded and theme objects
+      // are re-created.
+      const reloadedTheme = this.findThemeByName(this.activeTheme.name);
+      if (reloadedTheme) {
+        this.activeTheme = reloadedTheme;
+        return this.activeTheme;
+      }
     }
 
     // Fallback to default if no active theme or if it's no longer valid.
@@ -155,12 +255,20 @@ class ThemeManager {
     return this.getActiveTheme().semanticColors;
   }
 
+  private _getAllCustomThemes(): Theme[] {
+    return [
+      ...Array.from(this.settingsThemes.values()),
+      ...Array.from(this.extensionThemes.values()),
+      ...Array.from(this.fileThemes.values()),
+    ];
+  }
+
   /**
    * Gets a list of custom theme names.
    * @returns Array of custom theme names.
    */
   getCustomThemeNames(): string[] {
-    return Array.from(this.customThemes.keys());
+    return this._getAllCustomThemes().map((theme) => theme.name);
   }
 
   /**
@@ -169,7 +277,11 @@ class ThemeManager {
    * @returns True if the theme is custom.
    */
   isCustomTheme(themeName: string): boolean {
-    return this.customThemes.has(themeName);
+    return (
+      this.settingsThemes.has(themeName) ||
+      this.extensionThemes.has(themeName) ||
+      this.fileThemes.has(themeName)
+    );
   }
 
   /**
@@ -182,13 +294,11 @@ class ThemeManager {
       isCustom: false,
     }));
 
-    const customThemes = Array.from(this.customThemes.values()).map(
-      (theme) => ({
-        name: theme.name,
-        type: theme.type,
-        isCustom: true,
-      }),
-    );
+    const customThemes = this._getAllCustomThemes().map((theme) => ({
+      name: theme.name,
+      type: theme.type,
+      isCustom: true,
+    }));
 
     const allThemes = [...builtInThemes, ...customThemes];
 
@@ -232,7 +342,7 @@ class ThemeManager {
    * @returns A list of all available themes.
    */
   getAllThemes(): Theme[] {
-    return [...this.availableThemes, ...Array.from(this.customThemes.values())];
+    return [...this.availableThemes, ...this._getAllCustomThemes()];
   }
 
   private isPath(themeName: string): boolean {
@@ -249,8 +359,8 @@ class ThemeManager {
       const canonicalPath = fs.realpathSync(path.resolve(themePath));
 
       // 1. Check cache using the canonical path.
-      if (this.customThemes.has(canonicalPath)) {
-        return this.customThemes.get(canonicalPath);
+      if (this.fileThemes.has(canonicalPath)) {
+        return this.fileThemes.get(canonicalPath);
       }
 
       // 2. Perform security check.
@@ -288,7 +398,7 @@ class ThemeManager {
       };
 
       const theme = createCustomTheme(themeWithDefaults);
-      this.customThemes.set(canonicalPath, theme); // Cache by canonical path
+      this.fileThemes.set(canonicalPath, theme); // Cache by canonical path
       return theme;
     } catch (error) {
       // Any error in the process (file not found, bad JSON, etc.) is caught here.
@@ -318,13 +428,21 @@ class ThemeManager {
       return builtInTheme;
     }
 
-    // Then check custom themes that have been loaded from settings, or file paths
+    // Then check custom themes that have been loaded from settings, extensions, or file paths
     if (this.isPath(themeName)) {
       return this.loadThemeFromFile(themeName);
     }
 
-    if (this.customThemes.has(themeName)) {
-      return this.customThemes.get(themeName);
+    if (this.settingsThemes.has(themeName)) {
+      return this.settingsThemes.get(themeName);
+    }
+
+    if (this.extensionThemes.has(themeName)) {
+      return this.extensionThemes.get(themeName);
+    }
+
+    if (this.fileThemes.has(themeName)) {
+      return this.fileThemes.get(themeName);
     }
 
     // If it's not a built-in, not in cache, and not a valid file path,
diff --git a/packages/cli/src/ui/themes/theme.test.ts b/packages/cli/src/ui/themes/theme.test.ts
index b699893766..7240b04fa6 100644
--- a/packages/cli/src/ui/themes/theme.test.ts
+++ b/packages/cli/src/ui/themes/theme.test.ts
@@ -5,11 +5,15 @@
  */
 
 import { describe, it, expect } from 'vitest';
-import * as themeModule from './theme.js';
+import {
+  createCustomTheme,
+  validateCustomTheme,
+  pickDefaultThemeName,
+  darkTheme,
+  type Theme,
+} from './theme.js';
 import { themeManager } from './theme-manager.js';
-
-const { validateCustomTheme, createCustomTheme } = themeModule;
-type CustomTheme = themeModule.CustomTheme;
+import type { CustomTheme } from '@google/gemini-cli-core';
 
 describe('createCustomTheme', () => {
   const baseTheme: CustomTheme = {
@@ -152,7 +156,6 @@ describe('themeManager.loadCustomThemes', () => {
   };
 
   it('should use values from DEFAULT_THEME when DiffAdded and DiffRemoved are not provided', () => {
-    const { darkTheme } = themeModule;
     const legacyTheme: Partial<CustomTheme> = { ...baseTheme };
     delete legacyTheme.DiffAdded;
     delete legacyTheme.DiffRemoved;
@@ -170,12 +173,11 @@ describe('themeManager.loadCustomThemes', () => {
 });
 
 describe('pickDefaultThemeName', () => {
-  const { pickDefaultThemeName } = themeModule;
   const mockThemes = [
     { name: 'Dark Theme', type: 'dark', colors: { Background: '#000000' } },
     { name: 'Light Theme', type: 'light', colors: { Background: '#ffffff' } },
     { name: 'Blue Theme', type: 'dark', colors: { Background: '#0000ff' } },
-  ] as unknown as themeModule.Theme[];
+  ] as unknown as Theme[];
 
   it('should return exact match if found', () => {
     expect(
diff --git a/packages/cli/src/ui/themes/theme.ts b/packages/cli/src/ui/themes/theme.ts
index 5ba11cb32d..e95799b879 100644
--- a/packages/cli/src/ui/themes/theme.ts
+++ b/packages/cli/src/ui/themes/theme.ts
@@ -5,13 +5,19 @@
  */
 
 import type { CSSProperties } from 'react';
+
 import type { SemanticColors } from './semantic-tokens.js';
+
 import {
   resolveColor,
   interpolateColor,
   getThemeTypeFromBackgroundColor,
 } from './color-utils.js';
 
+import type { CustomTheme } from '@google/gemini-cli-core';
+
+export type { CustomTheme };
+
 export type ThemeType = 'light' | 'dark' | 'ansi' | 'custom';
 
 export interface ColorsTheme {
@@ -33,57 +39,6 @@ export interface ColorsTheme {
   GradientColors?: string[];
 }
 
-export interface CustomTheme {
-  type: 'custom';
-  name: string;
-
-  text?: {
-    primary?: string;
-    secondary?: string;
-    link?: string;
-    accent?: string;
-    response?: string;
-  };
-  background?: {
-    primary?: string;
-    diff?: {
-      added?: string;
-      removed?: string;
-    };
-  };
-  border?: {
-    default?: string;
-    focused?: string;
-  };
-  ui?: {
-    comment?: string;
-    symbol?: string;
-    gradient?: string[];
-  };
-  status?: {
-    error?: string;
-    success?: string;
-    warning?: string;
-  };
-
-  // Legacy properties (all optional)
-  Background?: string;
-  Foreground?: string;
-  LightBlue?: string;
-  AccentBlue?: string;
-  AccentPurple?: string;
-  AccentCyan?: string;
-  AccentGreen?: string;
-  AccentYellow?: string;
-  AccentRed?: string;
-  DiffAdded?: string;
-  DiffRemoved?: string;
-  Comment?: string;
-  Gray?: string;
-  DarkGray?: string;
-  GradientColors?: string[];
-}
-
 export const lightTheme: ColorsTheme = {
   type: 'light',
   Background: '#FAFAFA',
diff --git a/packages/core/src/config/config.ts b/packages/core/src/config/config.ts
index d432086155..0f027c989c 100644
--- a/packages/core/src/config/config.ts
+++ b/packages/core/src/config/config.ts
@@ -178,6 +178,57 @@ export interface AgentSettings {
   overrides?: Record<string, AgentOverride>;
 }
 
+export interface CustomTheme {
+  type: 'custom';
+  name: string;
+
+  text?: {
+    primary?: string;
+    secondary?: string;
+    link?: string;
+    accent?: string;
+    response?: string;
+  };
+  background?: {
+    primary?: string;
+    diff?: {
+      added?: string;
+      removed?: string;
+    };
+  };
+  border?: {
+    default?: string;
+    focused?: string;
+  };
+  ui?: {
+    comment?: string;
+    symbol?: string;
+    gradient?: string[];
+  };
+  status?: {
+    error?: string;
+    success?: string;
+    warning?: string;
+  };
+
+  // Legacy properties (all optional)
+  Background?: string;
+  Foreground?: string;
+  LightBlue?: string;
+  AccentBlue?: string;
+  AccentPurple?: string;
+  AccentCyan?: string;
+  AccentGreen?: string;
+  AccentYellow?: string;
+  AccentRed?: string;
+  DiffAdded?: string;
+  DiffRemoved?: string;
+  Comment?: string;
+  Gray?: string;
+  DarkGray?: string;
+  GradientColors?: string[];
+}
+
 /**
  * All information required in CLI to handle an extension. Defined in Core so
  * that the collection of loaded, active, and inactive extensions can be passed
@@ -199,6 +250,11 @@ export interface GeminiCLIExtension {
   resolvedSettings?: ResolvedExtensionSetting[];
   skills?: SkillDefinition[];
   agents?: AgentDefinition[];
+  /**
+   * Custom themes contributed by this extension.
+   * These themes will be registered when the extension is activated.
+   */
+  themes?: CustomTheme[];
 }
 
 export interface ExtensionInstallMetadata {