Nope - no theme at all! This blue is just how it looks out of the box. I have not even tried to download any - after all this is the browser that's only just barely tolerated because I need to test with it, not actually *use* it.
Opera is bad, too (they started that around version 6 or 7) - I just upgraded to the latest (12) to get the new 'Do Not Track' option (a bit late off the mark but they've caught up at least) but the upgrade threw out my 'native windows' theme and now it no longer adapts to the system style. Spent half an hour or so to find a theme that makes it look at least reasonable - but Opera is a browser that I occasionally actually use, and not merely to test with.