Implement color variables in CSS.
Summary: Added in color variables in most used places. Tweaked green to be a bit more serious.
Test Plan: Tested Tags, Error View, Timeline, Object Views, and Color Palette.
Reviewers: epriestley, btrahan
Reviewed By: epriestley
CC: aran, Korvin
Differential Revision: https://secure.phabricator.com/D6244