This website uses a Sass preprocessor, and gets most of its styles from GitHub's Primer design system.
All styles come from:
<link rel="stylesheet" href="/dist/index.css">index.css is a compiled version of index.scss, where all imported scss files are declared.
In general, we use Primer's utility classes as much as we can, and a 2E4A void writing custom styles whenever possible.
See styleguide.github.com/primer for reference.
We use Webpack to process the files - see javascripts/README.md for more information on how these files are compiled.