How to Change Login Screen Background Themes in XFCE Debian 11 Bullseye or Xubuntu 20.10. We can change the Login Screen Background, theme, icons, font very easily with lightdm-gtk-greeter-settings
so the first step we have to do is install it, I suggest you configure sudo first
sudo apt install lightdm-gtk-greeter-settings
next step open lightdm gtk+ greeter settings then type your root password
GTK+ themes and icon themes need to be installed in /usr/share/themes/ and /usr/share/icons/ directory so they are available in drop-down boxes in Appearance settings.
Now you can change Login Screen Background, theme, icons, window Position etc
Dont forget to save your setting
And then log out and then login again