I want to change the background for my login screen for 18.04.
I tried to change my login screen background via modifying /usr/share/gnome-shell/theme/ubuntu.css but that file does not exist.
I found nothing using Appearance either.
Can the background be changed?
I used it but it does not work.
It retained the same purple login screen.
Mine works. Changed the background to a locally stored wallpaper, logged out and there was my new background. Logged back in just fine. Good luck on this fixit7.
I found it required a reboot to work.
I have used various versions of Mate on and off from 19.10, 20.04, and now 20.10 - when I would click on right mouse button on the desktop, I would get the option to change the background. I have loaded images to that file location from WallPaperFlare.com and others too. Not sure if this can work for you since I have not used 18.04. Hope you can figure it out. 
All of our machines seem to vary. Glad to hear that your situation has improved fixit7. Keep warm.
Mate's login screen is green? How did you get a purple login screen?
Because he's propbably using Ubuntu since he was searching in /usr/share/gnome-shell/theme/ubuntu.css, Ubuntu MATE doesn't have gnome-shell.....
I have no idea, but the problem is solved.
The tool "Login Window" doesn't exist in control center of Ubuntu-MATE v22.10 at the time of writting this reply.
You can refer to:
how-to-change-login-screen-background-in-ubuntu-mate-21-04
Or:
how-to-change-login-screen-background
I think the latter include the easiest way.
