Multicolor headers

Summary: This adds a handful of 'Main Header' colors to change the look of Phabricator very slightly. I know I would probably set my dev header to a different color.

Test Plan: Tested each css class and color, can add more in the future.

Reviewers: epriestley, btrahan

Reviewed By: btrahan

CC: Korvin, epriestley, aran

Differential Revision: https://secure.phabricator.com/D7731