Modifying the look and feel of the GNOME 3 login screen seems like it is a lot harder than it should be, not least because the way these settings are stored seems to have changed at least twice since the original releases of the GNOME 3.
Changing the screen background
Should be simple, but the none of the theming options in ‘greeter.dconf-defaults’ appear to work. Can’t decide if this is because the way these settings are stored has changed or if it is a just a bug in GDM 3. (If I continue to use GNOME 3 then I’ll probably replace ‘gdm3’ with ‘lightdm’).
Hiding the list of users
By default GDM displays a list of users on the login screen, however I prefer to enter my username as well as the password. To hide the user list you need to edit the greeter defaults, then update the application configuration, and restart the display manager. To do this you need to be logged in as root, connect using ‘ssh’ from another system or use ‘Ctrl-Alt-F1’ to switch to a virtual console otherwise you will be logged out when you restart GDM!
# vi /usr/share/gdm/greeter.dconf-defaults
# Login manager options
# - Disable user list
# - Disable restart buttons
# - Show a login welcome message
Having updated the defaults you need to reconfigure and restart GDM.
# dpkg-reconfigure gdm3
# /etc/init.d/gdm3 restart
[ ok ] Restarting gdm3 (via systemctl): gdm3.service.