Install budgie-lightdm-theme by entering the following commands in the terminal:
sudo apt update sudo apt install budgie-lightdm-theme
LightDM Greeter theme for Ubuntu Budgie
This is the LightDM greeter theme and configuration used for a standalone Ubuntu Budgie installation. . This package can also be installed on other Ubuntu desktops desiring to use this theme, but it will replace the LightDM theme.