baffling error message

[Albert]

I get this same warning for all apps:

(xfwm4:2814): Gtk-WARNING **: Unable to locate theme engine in 
module_path: "xfce",

What is a 'module_path'?

I am running ROX desktop with xfwm4 as window manager.  I just 
upgraded from 4.0.1 to 4.2.2.  xfce-setting-show no longer works. 
  Now I have no way to configure xfwm4.  Is xfwm4 now married so 
tightly to xfce that it is no longer useful as a window manager 
for other desktops?