R1:0e5cd478c4bc

Move rgba rules into CelerityDefaultPostprocessor

Summary: Should make it simpler here to have more `rgba` rules in CSS for things like hovers, selected states. Maybe only use `rgb` colors? Color pallette probably needs an overhaul.

Test Plan: Bounce around random pages, buttons, menus. Everything appears normal.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D15273