feat(KB-173): add Zen theme to dashboard

- Add Zen to COLOR_THEMES array in core types
- Add Zen theme CSS styles with dark and light variants
- Add Zen theme option to ThemeSelector component
- Add tests for Zen theme selection in ThemeSelector
- Update index.html theme initialization to include Zen
- Add changeset for the new theme

packages/core/src/types.ts

@@ -15,6 +15,7 @@ export const COLOR_THEMES = [
   "ocean",
   "forest",
   "sunset",
+  "zen",
   "berry",
   "monochrome",
   "high-contrast",