Re-darken full width diff colors

Summary: Ref T12089. This reverts back to using "bright" colors for full width changes in green and red.

Test Plan:
Review diffs, see full width colors.

{F2333933}

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T12089

Differential Revision: https://secure.phabricator.com/D17171