By default KDM has this setting in /etc/kde4/kdm/kdmrc (Debian testing):

Code: Select all

Theme=@@@ToBeReplacedByDesktopBase@@@

In that mode actual theme is being set from /etc/default/kdm.d/10_desktop-base
Once you set up some theme in System Settings > Login Screen > Theme it modifies /etc/kde4/kdm/kdmrc and there is no apparent way to unset it (to switch it back to default). Is there any easy way to do it besides manually editing kdmrc?