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 avoid 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.