feat: Detect background color (#15132)

packages/cli/src/ui/themes/theme.ts

@@ -6,7 +6,11 @@
 
 import type { CSSProperties } from 'react';
 import type { SemanticColors } from './semantic-tokens.js';
-import { resolveColor, interpolateColor } from './color-utils.js';
+import {
+  resolveColor,
+  interpolateColor,
+  getThemeTypeFromBackgroundColor,
+} from './color-utils.js';
 
 export type ThemeType = 'light' | 'dark' | 'ansi' | 'custom';
 
@@ -499,3 +503,40 @@ function isValidThemeName(name: string): boolean {
   // Theme name should be non-empty and not contain invalid characters
   return name.trim().length > 0 && name.trim().length <= 50;
 }
+
+/**
+ * Picks a default theme name based on terminal background color.
+ * It first tries to find a theme with an exact background color match.
+ * If no match is found, it falls back to a light or dark theme based on the
+ * luminance of the background color.
+ * @param terminalBackground The hex color string of the terminal background.
+ * @param availableThemes A list of available themes to search through.
+ * @param defaultDarkThemeName The name of the fallback dark theme.
+ * @param defaultLightThemeName The name of the fallback light theme.
+ * @returns The name of the chosen theme.
+ */
+export function pickDefaultThemeName(
+  terminalBackground: string | undefined,
+  availableThemes: readonly Theme[],
+  defaultDarkThemeName: string,
+  defaultLightThemeName: string,
+): string {
+  if (terminalBackground) {
+    const lowerTerminalBackground = terminalBackground.toLowerCase();
+    for (const theme of availableThemes) {
+      if (!theme.colors.Background) continue;
+      // resolveColor can return undefined
+      const themeBg = resolveColor(theme.colors.Background)?.toLowerCase();
+      if (themeBg === lowerTerminalBackground) {
+        return theme.name;
+      }
+    }
+  }
+
+  const themeType = getThemeTypeFromBackgroundColor(terminalBackground);
+  if (themeType === 'light') {
+    return defaultLightThemeName;
+  }
+
+  return defaultDarkThemeName;
+}