Whether you're looking for icons to use in your ModelAdmins or StreamField, or looking for styles to overwrite, or simply interested in seeing what styles are used in the Wagatil's admin area, the styleguide is what you're looking for.
This is one of the easiest features in Wagtail to enable. Simply write one line of code, and it'll do everything else for you. Take a look a the code below.
The Git Commit
The entire git commit can be found here https://github.com/CodingForEverybody/learn-wagtail/commit/9226104eb57a12877baf6b0ece1a182c7c378fbb but truthfully, it's just one line of code that we added to dev.py. Only with Wagtail CMS can you enable something so quickly :P