Bug 876129 - 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 : ef4ac31003d75fb0fe17b493d7267ba3306e77bf