Fix header bgcolor on Workpanels
Summary: We use different background colors on <body> depending on ui.header. This makes the workpanel header transparent to fit all cases.
Test Plan: Switch to dark header color and check work boards for no UI glitches.
Reviewers: btrahan, epriestley
Reviewed By: epriestley
Subscribers: epriestley, Korvin
Differential Revision: https://secure.phabricator.com/D13658