update /theme to new slash command arch (#3791)

Co-authored-by: matt korwel <matt.korwel@gmail.com>
Co-authored-by: Abhi <43648792+abhipatel12@users.noreply.github.com>

packages/cli/src/ui/commands/themeCommand.ts

@@ -0,0 +1,16 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { OpenDialogActionReturn, SlashCommand } from './types.js';
+
+export const themeCommand: SlashCommand = {
+  name: 'theme',
+  description: 'change the theme',
+  action: (_context, _args): OpenDialogActionReturn => ({
+    type: 'dialog',
+    dialog: 'theme',
+  }),
+};