Restrict header Remarkup color to phui-source-sans only

Summary: These colors only apply to documents, not normal Remarkup (like tasks). Also fix an issue where headers and ToCs are first.

Test Plan: view a number of different layouts in Phriction, Maniphest.

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D12585