[Redesign] Fix some theme issues

Summary: Ref T8099, test a few themes and fix some inconsistencies.

Test Plan: Lots of changing headers, checking colors.

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T8099

Differential Revision: https://secure.phabricator.com/D13466