Misc Diffusion/Differential CSS tweaks

Summary: Various tweaks and fixes. Adds a File Contents view in Diffusion, normalizes spaces, colors.

Test Plan: tested differential and diffusion in my sandbox.

Reviewers: epriestley, btrahan

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Maniphest Tasks: T3952

Differential Revision: https://secure.phabricator.com/D7325