Add white as ActionHeader color, use on Dashboards
Summary: Trying to lessen the visual footprint of a heavy-widget dashboard. Adds a plain style.
Test Plan:
Tested my homepage and dashboards
Reviewers: epriestley
Reviewed By: epriestley
Subscribers: epriestley, Korvin
Differential Revision: https://secure.phabricator.com/D9454