Remove gradient on dropdown menus
Summary: Fixes T4732
Test Plan: Viewed a dropdown on a diff, now uses standard {$blue}
Reviewers: btrahan, epriestley
Reviewed By: epriestley
Subscribers: epriestley, Korvin, chad
Maniphest Tasks: T4732
Differential Revision: https://secure.phabricator.com/D8698