Since I prefer KDE for my desktop management, GTK application theme looks so ugly, it shows “grey” on Slackware Linux 13.0. I feels like back to 11 years ago :D. Whereas if I use Xfce, GTK application looks normal. So what’s the problem? Here we go, GTK application will read local configuration located on ~/.gtkrc or ~/.gtkrc-2.0 (for GTK2 application). So simply edit or make your config one.
vim ~/.gtkrc-2.0
and add this line
include "/usr/share/themes/your-preferred-theme/gtk-2.0/gtkrc"
mine is
include "/usr/share/themes/Clearlooks/gtk-2.0/gtkrc"
This will lead GTK application reads “/usr/share/themes/Clearlooks/gtk-2.0/gtkrc”, which is the configuration file of Clearlooks theme. The Clearlooks theme I got from gtk-engine package. But there are also default Xfce themes too in /usr/share/themes that ready to use. :-Q