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