Stop overriding color in differential, when we highlight specific changes.

Summary:
When we highlight specific changes (use the '.bright' css class), we override syntax highlighting with 'color:'.
This commit makes us stop doing that, by removing the 'color:'.

Test Plan:
My local instance sucks, so I can't test this :P @epriestley? :P

Reviewers:
epriestley

CC:

Differential Revision: 778