easy peasy. i always install gtk themes for system-wide availability. that is, i always place them in /opt/gnome/share/themes. themes usually come as a gz or a bz2, so unzip them into a temp location. then fire up a terminal and use 'su' to gain root privilages and then 'cp' to copy them to /opt/gnome/share/themes. actually, my linux is box is not connected to the internet, so i usually log in as root to use konquereror(in root, KDE is the one that i use) to copy the themes to /opt/gnome/share/themes....but thats not the most advisable method because its easy to screw up if logged in as root.
How do you actually install GTK themes in KDE?