Change background for image views in Differential

Summary:
Many images in Differential changesets are icons designed for
use on dark backgrounds. This makes them invisible on Differential's
white background. This adds an option to use a darker background
instead so you can see the images.

Currently this behavior is triggered on hover. (Also, it's a rather
garish fuchsia.) It seems fine UX-wise but I'm not totally sure of it.

Test Plan:
Load diff containing grippy_texture. Marvel at the grippy
fuchsia.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3272