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