This is Atom's UI library. Originally forked from Bootstrap 3.3.6, then merged with some core styles and now tweaked to Atom's needy needs.
Note: This package gets bundled with Atom and isn't meant to be used outside of the Atom editor. It depends on variables provided by Atom themes.
Here a list of all components. Open the Styleguide package (cmd-ctrl-shift-g) to see them in action and how to use them.
If you need something, feel free to open an issue and it might can be added. ✌️