[Redesign] Darken things when dark header is used

Summary: Ref T8099, uses original bgcolor when dark headers are used.

Test Plan: Switch between light and dark Phabricator, see new colors.

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T8099

Differential Revision: https://secure.phabricator.com/D13361