How to change the ubuntu login screen

readme.md

$ sudo gedit /usr/share/gnome-shell/theme/ubuntu.css

Find "#lockDialogGroup" keyword:

Replace with:

#lockDialogGroup {
  background: #2c001e url(file:///[fileLocation/filename.png]);
  background-repeat: no-repeat;
  background-size: cover;
  background-position: center;
}

Save and restart the system