Make notifications visually clearer, like Feed

Summary: See downstream https://phabricator.wikimedia.org/T166358. The notifications menu is missing some CSS to color and style values in stories like "renamed task from X to Y".

Test Plan:
Before:

After:

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D20310