Tweak dialog header, mask color

Summary: A little more pop in the mask color (light to dark) and some more space in the dialog header.

Test Plan: Visit dialogs on page and in actual dialogs.

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D12660