feat(extensions): add support for custom themes in extensions

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.

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"
+      }
+    }
+  ]
+}

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);
+  });
+});

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];
 

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 {

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';