Hi, Dragan,
you're right! I just switched to the default twentyeleven theme for a test and voilà: the plugin displays correct: http://awesomescreenshot.com/04flodkbf So my assumption was right, the theme's CSS seems to be botched. Unfortunately I'm not *that* CSS expert - I was hoping you'd be able to give me some pointers on what to look at... nevermind, I'm going to ask the theme guys, too.
Anyway, thanks for the idea of switching the theme (why didn't I think of that in the first place?) and for the tip on the cool screenshot-tool for firefox ;-)
Kind regards,
Henning