Add a few more high contrast CSS rules
Summary: In new PHUITwoColumnLayout, some new colors were used that aren't in the High Contrast PostProcessor. Adding them and proper fallbacks.
Test Plan: High Contrast on/off on a timeline page.
Reviewers: epriestley
Reviewed By: epriestley
Subscribers: Korvin
Differential Revision: https://secure.phabricator.com/D16515