Thanks to all for your hints.
@Blaazen: Since themes are limited to QT4, I discardrd this solution. But, using themes and themesservices was a complete new programming ascpect for me; i never used it before. Very interesting.
@WP: TEditButton works as expected on the Mac, no problems. Simple solution, easy to use.
@jamie: Very good idea, looks nice and also easy to implement. This could become my way.