Optional body class for dust background.
Summary: This adds an option dust background for certain application designs, like Macro and Pholio to help make the list views pop more.
Test Plan: Reviewed Macro and Pholio.
Reviewers: epriestley
Reviewed By: epriestley
CC: aran, Korvin
Differential Revision: https://secure.phabricator.com/D5209