R1:7aeefc0cca5d

Add an Experimental Dark Mode to Phabricator

Summary: Mostly this is an exercise to clean up our CSS and Celerity processor by making sure all important color decisions are generatable. It's somewhat resonable to use if you don't review code. Posting it up here mostly so I don't lose the work.

Test Plan: Visit lots and lots of pages with dark mode on and off.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D18227