Your gui windows look as good as the amount of effort you put into them. Most people stop working once they get good functionality, but add a bit more time and you can have beauty as well.

You can always make gui's with a nice graphics background if you use the Tk canvas, and just look at the Tk::Zinc demo for great looking graphics, with transparency and bezier (curved lines). If you want automatic rounded entry boxes, etc., you can go with Gtk2 and it's styles. Tk is supposed to get styles soon.