Install lightdm-gtk-greeter by entering the following commands in the terminal:
sudo apt update sudo apt install lightdm-gtk-greeter
Description:
simple display manager (GTK+ greeter)
A LightDM greeter that uses the GTK+ toolkit.
Homepage: https://launchpad.net/lightdm-gtk-greeter
Version: 2.0.5-0ubuntu1
Section: universe/x11