This adds a fer extra colors that work well in dark mode, light should still be
fixed.
Also fixes up a few other visual elements:
- Line numbers should line up with lines of code again.
- The buttons to switch to a different branch at the top of a repository page
looks a little more in-place. It has the same padding as the other form
elements, and the same background as the rest of the header.
- Give other form elements on repository pages more consistent padding.
- Separate the ‘log’, ‘stats’, and ‘plain’ buttons on the repository tree page
so they don't just look like they're one word together.
