Adding style to TopWindow makes sense, I have improved a little bit upon your version (it does not need to be Color, Value is more flexible as it allows e.g. putting image there).
HOWEVER, the original claim that background is black is quite weird - I see no such behaviour (and I am using Win8 too, plus apps deployed do too). Have you checked with latest sources (svn/nightly build)? If yes, can you elaborate on your Win8 setup or anything that could help me reproduce this undesirable behaviour? (perhaps some screenshots would help too). Does even theide have black backgrounds?