Include default CSS in docs?

[TomJGooding]

Obviously all the DEFAULT_CSS is available by checking the source code, but perhaps this should somehow be included in the docs? If the default CSS was more discoverable, this might help reduce the number of issues/questions about styling.

[TomJGooding]

Apparently I didn't search far back enough in the issues, as this has already been discussed in #1705 (comment)

I'll keep this as a stand-alone issue for now, but feel free to close if considered a duplicate.

[TomJGooding]

I think this would be even more useful following the changes with themes.

You really need to understand all the variables in the default CSS to correctly change the styling. (Honestly, even after checking the source for the default CSS I now struggle a bit.)