Remove dark Phabricator special CSS

Summary: Looks like the border was almost always overridden and the background was only slightly darker. Safe to remove these. Fixes T10424

Test Plan: Set Workboard to "red" see red workboard. Toggle CSS on and off on homepage, note little to no visual difference.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T10424

Differential Revision: https://secure.phabricator.com/D15331