[CSS] Unnecessarily large CSS file

[mwskwong]

Apparently @radix-ui/themes/styles.css includes every single CSS for the library, including unused components and color palettes. Heck even the theme panel CSS is included, while it is supposed to be a dev-only component. This creates a 700KB CSS file (uncompressed) in a "hello world" app in PROD build.

Proposal

Create separate imports for each item

• Core (utils + non-color related design tokens) - expected to be imported in root
• Split color palettes (with light and dark mode variants) into individual CSS files - import when needed
• Split components into individual CSS files - import when needed