primer/css ? reverse-engineered prompt

Build me a CSS design system for a GitHub style product, using SCSS and reusable utility classes.

I want it to feel clean, consistent, and easy to drop into other projects, with a main entry file and smaller modules I can import on their own when needed. Include the basics a design system needs, like colors, spacing, typography, layout helpers, and common UI building blocks, plus a simple way to theme things. Make sure the styles are organized well, documented, and easy to maintain.

Please also include a small docs setup or examples so someone can see how to use the classes and import the Sass files. If there are older patterns that should not be used anymore, keep the current ones working and make the newer approach clear. Look up current docs online if you need to, and make it feel polished enough to ship as a real library.