Being able to split the CSS into separate files, as well as being able to even further break them down into blocks of space-style with markdown between (Jupyter Notebook style) is a hidden super power of SilverBullet, I’d say.
I pretty much went element by element by inspecting it with the browser dev tools and tweaking the values. I don’t think there is dedicated documentation to themeing – or I haven’t found it at least 
Top Bar Icons are: (some with custom icons)
- Home
- Search
- Open Daily Note
- Open Page (Ctrl+K)
- Run Command (Ctrl+Shift+/)
- Toggle Tree View
- Create Subpage (source)
- Delete Page
- Settings