Update Chatlog CSS, fix standard backgrounds

Summary: Uses standard colors in Chatlog, updates the background colors in UIExamples

Test Plan: View UIExamples page, check hex values, check Chatlog new CSS

Reviewers: epriestley, btrahan

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Differential Revision: https://secure.phabricator.com/D8171