Add theming support.

- Added a number of common themes to our support matrix:
 - AtomOneDark
 - Dracula
 - VS
 - GitHub
 - GoogleCode
 - XCode
 - ... Admittedly these all were randomly picked, we could probably curate these better...
- Added a new `ThemeDialog` UI that can be accessed via `/theme`. It shows your currentlyt available themes and allows you to change them freely. It does **not**:
 - Save the theme between sessions
 - Allow you to hit escape
 - Show a preview prior to selection.
- These themes are from reacts highlight js library.

Fixes https://b.corp.google.com/issues/412797985

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

@@ -4,19 +4,64 @@
  * SPDX-License-Identifier: Apache-2.0
  */
 
+import { AtomOneDark } from './atom-one-dark.js';
+import { Dracula } from './dracula.js';
+import { GitHub } from './github.js';
+import { GoogleCode } from './googlecode.js';
+import { VS } from './vs.js';
 import { VS2015 } from './vs2015.js';
+import { XCode } from './xcode.js';
 import { Theme } from './theme.js';
 
+export interface ThemeDisplay {
+  name: string;
+  active: boolean;
+}
+
 class ThemeManager {
   private static readonly DEFAULT_THEME: Theme = VS2015;
   private readonly availableThemes: Theme[];
   private activeTheme: Theme;
 
   constructor() {
-    this.availableThemes = [VS2015];
+    this.availableThemes = [
+      AtomOneDark,
+      Dracula,
+      VS,
+      VS2015,
+      GitHub,
+      GoogleCode,
+      XCode,
+    ];
     this.activeTheme = ThemeManager.DEFAULT_THEME;
   }
 
+  /**
+   * Returns a list of available theme names.
+   */
+  getAvailableThemes(): ThemeDisplay[] {
+    return this.availableThemes.map((theme) => ({
+      name: theme.name,
+      active: theme === this.activeTheme,
+    }));
+  }
+
+  /**
+   * Sets the active theme.
+   * @param themeName The name of the theme to activate.
+   */
+  setActiveTheme(themeName: string): void {
+    const foundTheme = this.availableThemes.find(
+      (theme) => theme.name === themeName,
+    );
+
+    if (foundTheme) {
+      this.activeTheme = foundTheme;
+    } else {
+      throw new Error(`Theme "${themeName}" not found.`);
+    }
+  }
+
   /**
    * Returns the currently active theme object.
    */