Tweak colors on Conpherence Notification Menu

Summary: Fixes T3690. Uses standard colors, smaller borders.

Test Plan: Review Menu with and without a notification

Reviewers: epriestley, btrahan

Reviewed By: btrahan

CC: Korvin, aran

Maniphest Tasks: T3690

Differential Revision: https://secure.phabricator.com/D6710