Install arctica-greeter-theme-debian by entering the following commands in the terminal:
sudo apt update sudo apt install arctica-greeter-theme-debian
LightDM Arctica Greeter - Debian Theme
A greeter shell for the LightDM login manager. Arctica Greeter can be used as local display manager as well as thin client login manager. . This package themes Arctica Greeter with a Debian look'n'feel.