feat(ui): Introduce useUI Hook and UIContext (#5488)

Co-authored-by: Jacob Richman <jacob314@gmail.com>

packages/cli/src/core/theme.ts

@@ -0,0 +1,21 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { themeManager } from '../ui/themes/theme-manager.js';
+import { type LoadedSettings } from '../config/settings.js';
+
+/**
+ * Validates the configured theme.
+ * @param settings The loaded application settings.
+ * @returns An error message if the theme is not found, otherwise null.
+ */
+export function validateTheme(settings: LoadedSettings): string | null {
+  const effectiveTheme = settings.merged.ui?.theme;
+  if (effectiveTheme && !themeManager.findThemeByName(effectiveTheme)) {
+    return `Theme "${effectiveTheme}" not found.`;
+  }
+  return null;
+}