I recently installed the Xubuntu desktop environment on my Ubuntu machine. It's nice, but I switched back to Ubuntu as by default desktop environment. But when I change the volume or brightness, the HUD icons are still from the XUbuntu theme. Also, when I shutdown, the splash screen says Xubuntu.
I'm quite sure that I am in fact running Ubuntu:
$ echo $DESKTOP_SESSION ubuntu
and in /etc/lightdm/lightdm.conf