Tweaks form background colors.
Summary: This updates the shaded views used in Maniphest and Differential to a more blue/grey to better match the table headers and breadcrumbs.
Test Plan: review colors in photoshop and in multiple apps. TODO: update panel filter colors (coming next)
Reviewers: epriestley, btrahan
Reviewed By: epriestley
CC: aran, Korvin
Differential Revision: https://secure.phabricator.com/D4447