"Theme" itself is rather common in german lingo (spread from wordpress probably), "Skin" and "Stil" [Style] have pretty much died out. The other more german sounding terms seem either foreign or plain wrong. Of those I'd go with "Aussehen", if a non-english term is absoloutely necessary.
@vanitasvitae that's the literal translation, though it actually means "Topic"
@kingu automated translation not helping here