Bug 865806 - Part 2: Prefer to look at <body> instead of <html> when determining whether the background color of a page is dark or not; r=roc

This seems to be a slightly better heuristic to follow the CSS Rendering code
in all cases, since the background color for the body element often overrides
that of the html element.

--HG--
extra : rebase_source : 6c0dba10c1ac4f08914632179e15c13c2ff271aa