radix-ui/themes — reverse-engineered prompt

Build me a polished open source React UI theme library that makes it fast to put together good looking apps without fighting styling all day. I want it to feel like a real design system, with reusable accessible components, clean defaults, consistent spacing and typography, and easy theme customization for colors, radius, shadows, and light and dark mode.

Please make it feel production ready and easy to maintain, not just a random component demo. The components should be keyboard friendly, screen reader friendly, and visually consistent across the whole system. Include a simple docs or demo site where someone can browse the components, see examples, and understand how to use the themes in a normal app.

Keep the API straightforward for developers, and make the whole thing feel cohesive and thoughtfully designed. If there are existing patterns or docs for this project online, feel free to look them up so the result matches the intended style and quality.