[Redesign] Tighten up Remarkup / PHUIFontKit

Summary: Ref T8099, Tightens up lists, normalizes header colors.

Test Plan: Review Phriction / Diviner

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T8099

Differential Revision: https://secure.phabricator.com/D12975