Allow setting of button colors in headers

Summary: We currently override button color for headers, since the default is blue, but if a developer sets a specific color, we should respect that.

Test Plan: Set a button in the header to green and see green. See grey everywhere else.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D17922