Theme overhaul

The general theme overhaul as discussed in #193 (closed) and other issues.

_themes.scss is now finished, next up will be to add mixins for colors outside of the _themes.scss file.

I've also kept a doc with info (mostly for myself) but it's got a few notes in there too for the future: https://gitlab.com/snippets/1742005.

Fixes #193 (closed).