option to set configurable background image for the skin as a whole
Defaults to the cat, but can ideally be set to anything appropriate
to the wiki in question. Recommendation is a 20-50% opacity black/
white background image, where the alpha values of the b/w parts are
likewise around 10-50/255.
Bug: T193330
Change-Id: Ica3e2ce2568557ef157d06d1b7127b030f3c38f0
Isarra