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