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

{F164709}

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9454