As System > Administration > Login Screen doesn't offer so many options anymore, the way login screen looks can be changed like this:
1. logout of your current session and return to login screen
2. get your command line interface (CLI) by pressing Ctrl+Alt+F1
3. login using your account, or root account.
4. type export DISPLAY=:0.0 for the $DISPLAY variable to be available to users
5. type sudo -u gdm gnome-control-center - this will open up gnome control center for "gdm" user; this is the user that is used for the login screen.
6. switch to graphical display using Ctrl+Alt+F7
7. configure theme, background and whatever else you want for the login screen
8. close gnome-control-center and login using your account (you should already have the new theme at your login screen)