Problem: You’re tired of the default Kubuntu login theme and want to install a new one. But when you try to add new themes by going to System Settings > Login Screen (under the category System Administration) > Theme, nothing happens.
Cause: A flaw in KDE
Solution: Open Konsole and type:
kdesudo kcmshell4 kdm
This will open the Login Screen module, but as root. Now try installing a login theme. You should be able to see the newly installed theme in the list once you install, and be able to specify it as the default.