Lighten up red and green in diffs

Summary: Because new+old stack, these colors were darker than intended. Lightening them up a little bit.

Test Plan: Review a diff with new + old code and addtion subtractions.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D15994