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

Test Plan:
Tested prose diffs, email prose diffs, and a regular Differential revision.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T12060

Differential Revision: https://secure.phabricator.com/D17138