Flatten button colors, remove black

Summary:
Simplifies buttons states and colors.

Test Plan: Visited many pages. Seems like I didn't break anything. IE, Safari, FF, Chrome.

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D12644