As a KDE user I got pretty tired of the ugly default looks of GTK applications (and also WX applications in Linux, they have the same looks). They looked cool in 1997, but these times are over now. And all the time I was too lazy to find out how I could theme GTK, but I didn't realize it was really simple. These are the steps on a Gentoo machine:

1. First a new GTK theme is installed. This is a special one, because it uses Qt for all drawing operations.
   
   emerge -av x11-themes/gtk-engines-qt