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