The majority of the CSS in the application is written in SASS. There are a few places in the code base that have style blocks in ERB templates, for inlining critical CSS (good). There are also some styles that live in ERB templates that are not critical CSS (bad). That is a bit of refactoring that needs to be done. PRs welcome!
Important files when working with SASS in the project:
SASS is compiled and served using Sprockets which packages static assets in Rails.
For more about branding, theming or design in general in regards to DEV, refer to the Design Guide documentation.