Remove phui-theme sidebar colors

Summary: Removes the special CSS to change the sidebar when the header is called. Also switched to using a more standard background color. I'd also like to make collapsed the default, but can't find best way to do that.

Test Plan: Vist profile, projects, collapse and open nav, try a workboard color. Still all good.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D16673