How do I install themes on openbox?
When Obconf is open, look for “Themes” and click on it. From there, just scroll through the list and click on any of the items in the list. Openbox should switch to it instantly.
How do I change the theme on my GTK Openbox?
Use the Gnome settings.
If you add 'gnome-settings-daemon &' to your autostart file (see below), all Gtk applications will use the Gtk theme, icons and fonts specified in the Gnome settings dialog. To change these, just start 'gnome-control-panel' (To make things easy, add an entry in your Openbox menu for this).