Make base font a Celerity CSS variable
Summary: For consistency, we switch back to base font in a few places when using alternate fonts like source-sans or monospace, this makes sure the base font is consistently reset.
Test Plan: Review a Document, a Diff, and a Legalpad form
Reviewers: btrahan, epriestley
Reviewed By: epriestley
Subscribers: Korvin, epriestley
Differential Revision: https://secure.phabricator.com/D11891