Update diff highlight colors for better color blindess distinction
Summary: Tweaks the diff colors here a bit, as well as making full diffs slightly easier to read in full. Ref T12060
Tested prose diffs, email prose diffs, and a regular Differential revision.
Reviewed By: epriestley
Maniphest Tasks: T12060
Differential Revision: https://secure.phabricator.com/D17138