Darkmode style refactoring and tweaking

Refactoring:
* Refactor CSS to make clear what varies with light/dark mode and what doesn't.
* Consistently use 2 spaces for indenting in CSS file.
* Remove unused Chroma CSS classes to make styling easier to understand.
* Add some comments about how Chroma CSS classes map to Go code.
* Organize Chroma CSS classes so that like-styled classes are contiguous.
* Display "operators" like regular code - doesn't make sense to style ":="
  specially but not "=" e.g.

Style tweaks:
* Adjust dark mode styling to be less blue, more subdued.