This script changes the login screen's background color or background image of Ubuntu 20.04. LTS.
Download the script focalgdm3.sh with curl -L -O https://raw.githubusercontent.com/cmar0ck/focalgdm3/master/focalgdm3.sh.
Then run the command sudo bash focalgdm3.sh --set
and follow the prompts.
To reset everything the script changed
run the command sudo bash focalgdm3.sh --reset
That's it.