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