Lots of little fixes for Dark Mode (Experimental)

Summary: Cleans up a bunch of Differential odd/special colors. Adds some basic "highlight" colors instead of pure yellow.

Test Plan: Test each color change in normal and dark modes.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D18239