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
2. Go to the KDE Control Centre -> Appearance and Themes -> GTK Styles and Fonts.
3. Make sure that the options Use my KDE style in GTK applications and Use my KDE fonts in GTK applications are selected.
4. Restart KDE to apply the changes.

Besides I chose the Nuvola_FF2 theme to make it look (almost) exactly like Konqueror: