docs: custom themes in extensions (#19219)

docs/extensions/index.md

@@ -1,8 +1,8 @@
 # Gemini CLI extensions
 
-Gemini CLI extensions package prompts, MCP servers, custom commands, hooks,
-sub-agents, and agent skills into a familiar and user-friendly format. With
-extensions, you can expand the capabilities of Gemini CLI and share those
+Gemini CLI extensions package prompts, MCP servers, custom commands, themes,
+hooks, sub-agents, and agent skills into a familiar and user-friendly format.
+With extensions, you can expand the capabilities of Gemini CLI and share those
 capabilities with others. They are designed to be easily installable and
 shareable.
 

docs/extensions/reference.md

@@ -172,6 +172,9 @@ The file has the following structure:
   `"excludeTools": ["run_shell_command(rm -rf)"]` will block the `rm -rf`
   command. Note that this differs from the MCP server `excludeTools`
   functionality, which can be listed in the MCP server config.
+- `themes`: An array of custom themes provided by the extension. Each theme is
+  an object that defines the color scheme for the CLI UI. See the
+  [Themes guide](../cli/themes.md) for more details on the theme format.
 
 When Gemini CLI starts, it loads all the extensions and merges their
 configurations. If there are any conflicts, the workspace configuration takes
@@ -302,6 +305,50 @@ extension's root folder and add your agent definition files (`.md`) there.
 Gemini CLI will automatically discover and load these agents when the extension
 is installed and enabled.
 
+### Themes
+
+Extensions can provide custom themes to personalize the CLI UI. Themes are
+defined in the `themes` array in `gemini-extension.json`.
+
+**Example**
+
+```json
+{
+  "name": "my-green-extension",
+  "version": "1.0.0",
+  "themes": [
+    {
+      "name": "shades-of-green",
+      "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"
+      }
+    }
+  ]
+}
+```
+
+Custom themes provided by extensions can be selected using the `/theme` command
+or by setting the `ui.theme` property in your `settings.json` file. Note that
+when referring to a theme from an extension, the extension name is appended to
+the theme name in parentheses, e.g., `shades-of-green (my-green-extension)`.
+
 ### Conflict resolution
 
 Extension commands have the lowest precedence. When a conflict occurs with user

docs/extensions/writing-extensions.md

@@ -21,6 +21,7 @@ Extensions offer a variety of ways to customize Gemini CLI.
 | **[Context file (`GEMINI.md`)](reference.md#contextfilename)** | A markdown file containing instructions that are loaded into the model's context at the start of every session.    | Use this to define the "personality" of your extension, set coding standards, or provide essential knowledge that the model should always have.                                                                                                                                                | CLI provides to model |
 | **[Agent skills](../cli/skills.md)**                           | A specialized set of instructions and workflows that the model activates only when needed.                         | Use this for complex, occasional tasks (like "create a PR" or "audit security") to avoid cluttering the main context window when the skill isn't being used.                                                                                                                                   | Model                 |
 | **[Hooks](../hooks/index.md)**                                 | A way to intercept and customize the CLI's behavior at specific lifecycle events (e.g., before/after a tool call). | Use this when you want to automate actions based on what the model is doing, like validating tool arguments, logging activity, or modifying the model's input/output.                                                                                                                          | CLI                   |
+| **[Custom themes](reference.md#themes)**                       | A set of color definitions to personalize the CLI UI.                                                              | Use this to provide a unique visual identity for your extension or to offer specialized high-contrast or thematic color schemes.                                                                                                                                                               | User (via /theme)     |
 
 ## Step 1: Create a new extension