ProgressUI is the user experience framework for building beautiful, clean, and nimble campaign and non-profit applications. ProgressUI is built and maintained by the Design Team at EveryAction / NGP VAN. ProgressUI takes cues from Bootstrap and Google Material Design, including our documentation which is a derivative work used under CC BY 3.0 of both. If you are familiar with Bootstrap, you should have no trouble ramping up on ProgressUI.
If you are not going to be changing any of the styles inside ProgressUI, and just want to import it into your project, your best bet will be to download the zipped repo here. After it's finished downloading navigate down into app/assets/stylesheets, and pull out progress.min.css. Copy-paste the stylesheet <link> into your <head> before all other stylesheets to load ProgressUI.