## Problem
- We still use/maintain the legacy theming even though we moved to
Shadcn
- We have two themes list with similar code
## Solution
- Migrate some components to `cva` and `cn` instead of `styleHandler`
- Remove redundant `themes` in favor of `singleThemes`
<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit
* **Bug Fixes**
* Standardized theme selection across the app so theme menus
consistently offer the same light/dark/system options, including
dropdowns, navigation menus, and command menus.
* **Refactor**
* Refreshed styling for multiple shared UI components (menus, tabs, side
panels, loading indicators, icons, and modal dialogs) to improve
consistency while preserving visual behavior.
* **Chores**
* Improved the UI patterns CI “validate exports” check for clearer
failure output when exports change unexpectedly.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->
* Add shimmering-loader CSS to ui-patterns.
* Import the shimmering-loader classes from the ui-patterns component.
* Remove ShimmeringLoader from studio.
* Migrate studio to use ui-patterns/ShimmeringLoader.
* Migrate away from using default import for ShimmeringLoader.
* Fix the css imports in docs and studio.
* Move all components in ui-patterns into src folder. Add exports field for all files.
* Fix all apps to use the new ui-patterns.
* Fix the design-system build.
* Remove all unused rexports from ui-patterns index. Apps should use direct imports instead.
* Change the tailwind content property to include src folder of ui-patterns.
* Remove autoprefixer from the tailwind configs.
* Remove autoprefixer as a dependency.
* Fix the CVA conditions in FormLayout.
