Primer CSS
The CSS design system that powers GitHub
...Styles can be mixed and matched to achieve many different layouts, independent of their location. These styles fall into three categories. Single-purpose, immutable styles, that do one thing well. Abstracted patterns for frequently used visual styles. Primer CSS is built upon systems that form the foundation of our styles such as spacing, typography, and color. This systematic approach helps ensure our styles are consistent and interoperable with each other. The base-8 spacing scale is highly composable and works with the density of GitHub’s content. Margin and padding spacers bring consistency to vertical and horizontal rhythm.