In order to get a somewhat working Xfce installation into a custom prefix like /home/user/blah, GTK_PATH needs to be set in order to let GTK find the Xfce theme engine. It should point to $libdir, something like GTK_PATH="$GTK_PATH:/home/user/blah/lib/gtk-2.0" Without setting this, GTK won't find the Xfce theme engine. (There are alternative ways like symlinking libxfce.so into ~/.gtk-2.0/engines/ but this requires direct user interaction. Furthermore, since stuff like LIBGLADE_MODULE_PATH is also set in xinitrc, it would be just straightforward to also set GTK_PATH.)
Makes sense.
Fixed in revision 29480: Export GTK_PATH variable in xinitrc so that GTK+ has a chance to find the Xfce theme engine even in a custom prefix (bug #4925).