Yeah many of the media player graphics are unthemeable, including the now playing shortcut. I'd file a bug with Nokia, but it seems like they don't even respond to bug reports anymore, much less actually fix them.

The fullscreen media player controls seem to just reuse the regular media player button graphics (with bad cropping applied for no apparent reason). I could fix that by making the buttons smaller (i.e. just the symbols, without the rounded blue rectangle behind them). I don't really want to release an update just for that, but I'll probably change it if/when I get some more bug reports to fix at the same time.