More minor Remarkup tweaks

Summary:
Fixes T7444, Fixes T7963. A few small changes:

• Section headers are slightly larger
• Regular headers are slightly darker
• hr's have more padding, standard blue color
• code blocks now have correctly attached header colors
• Phriction emdeded links have color anchor color

Test Plan: Lots of various pages in Phriction, Diviner.

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7963, T7444

Differential Revision: https://secure.phabricator.com/D12596