It's great that you posted this, because I literally have a tool that can help you build a brand new UI with any design system you like, in your AI agent, in one click.
I saw that Igor dropped context.dev in the comments for design md extraction, and, honestly, it's really good, it extracts everything and works amazing as a platform.
If you'd like to try something else, I made a free chrome extension called UIDrop that "snaps" a website - takes a screenshot for visual reference and then outputs the tokens, which then get parsed either into a prompt or a "system.md". In one click, you send that to Gemini and you can start building. Let me know if it's useful: uidrop.site/