Edit greenfoot.defs (or greenfoot.properties) with a text editor, uncomment 'bluej.lookAndFeel=crossplatform' (i.e. remove the '#') (for greenfoot.properties: add this line), and proceed to enjoy the horrible "Metal" look-and-feel. (There's a reason why we use Nimbus).
Oh well, i do not like the nimbus-Theme, it is to Mac-like.
Big thx for the help! :D