i couldn't change the background in the login screen , after a lot of try and error , i found that the .gresource file responsible for it was in /usr/share/themes/ZorinBlue-Dark/gnome-shell/gnome-shell-theme.gresource .
at the end i extracted the files , changed the background image and recompile the new package which was a half-success , because when i log-out i found my desired image , i couldn't log-in . second , when i restarted the pc i couldn't use gnome at all ( automatic log-in) (error message from gnome : i didn t write it ) . i fix it with live boot and using timeshift .
but the probleme : how to change the background .
notice : GDM settings and tweaks didn't work (no error appeared)
//sorry if i sound agressive and for the grammer mistakes ( english is my third language ) . i am new to linux about ( two monthes btween old laptop to VM and finally my real laptop ) .
Would you be able to use this gnome extension to do what you're looking to do?
Lock screen background - GNOME Shell Extensions
It seems like it should be supported and should work (I'm assuming you're on Zorin 17?).
Generally I try to avoid manually going in and changing things, gnome has so many things tied together that it's quite easy to break (as you've figured out 
).
Welcome to the Forum!
You could try something different:
When You go to /usr/share/themes/ZorinBlue-Dark/gnome-shell/assets/ You should find there a File called login-background.png
So, take the Picture that You want use in .png File Format and name it login-background.png and replace the one in the assets Folder with Your Picture with the same Name and then try it. Important is here that it really have the same Name and is a .png File.
Are you using the flatpak version of GDM Settings?
thanks for the reply . i can install it but no luck in changing the background after clicking log out .
thanks for the reply . i didn t understand : are you talking about changing the image in the folder ( because this didn t work ) or are you talking about recompile it into .gresource file and move it ( isn t this the same with what i did?) .
thanks for the reply .
yes , i didn t find another version in the software app . and when i go to the github page , the versions are :
- [AppImage] ( which i didn t install it because of the developer mentioning sandboxes )
- [Flatpak]
- [ArchLinux/Manjaro]
- [Gentoo Linux]
- [Manual Installation]
Yes, I mean this. When You would take a .png Picture File and name it like the one in the Folder and then delete and replace the one in the Folder, does this not work?
thanks for replying . and yes , it didn't work (as i said , the file responsible for the login screen is /usr/share/themes/ZorinBlue-Dark/gnome-shell/gnome-shell-theme.gresource . ) .
Yes, that is the File that makes the Login Screen looks like it should. But some of the Components are in the asstes Folders. And in the File You can find the Filename login-background.png. So, it takes the File from the assets Folder. when you now would replace it with another Picture File in .png Format and with the same Name it should take it.
Maybe I try it at the Weekend by myself. I hope, I don't break my System, hahaha!
thanks for replaying again , do you have some documentation i could refere to ?
No, but when I have tested it and it really should work, I can give You a Step by Step Guide.
thanks again , and yes i really want the step by step guide
Unfortunately bad News. I replaced the Pictures like I wanted to do and that went without Problems. But the Picture on the Login Screen didn't changed. I replaced the Files in the Dark and Light Version, but didn't helped. Which is weird because the Picture wasn't there; there were only my Picture ... I did something similar in the Past with a GRUB Theme. So, I thought it should be work here, too. But it is how it is.
I know from a Program called Loginized which is to customize the GDM Login Screen but I didn't tested it. And it is pretty old. So, if this works with Gnome 43 on Zorin ...
Sorry for making false Hope's.
thanks for your efforts anyway . it is not your probleme so i'm really thankful .
I feel your pain. I have been down this road in trying to customize Gnome many times... What used to work, suddenly no longer does. What should work does not work.
Gnome is a firm believer that the user should not control UI Appearance. You can read much about this on Tobias Bernards blog.
I looked at themes I created for Gnome in the past - and that is indeed where they are stored: /usr/share/themes/THEME-NAME/gnome-shell/assets I checked with @Storm's themes. Same result.
The .css in the Zorin Gnome theme calls on #lockDialogGroup
The theme is wrapped as a gresource file, which must be unpacked, then repacked - which I do not remember off the top of my head how to do, but is likely simple with guides on the web that tell how.
Changing the .css from
#lockDialogGroup
resource:///org/gnome/shell/theme/login-background.png
to
#lockDialogGroup
resource:///org/usr/share/backgrounds/NEW-IMAGE-NAME.png
and placing your wallpaper in usr/share/backgrounds (be sure to adjust the above to the actual new image name), repacking the gresource, clearing the Shell Cache and restarting the service may work:
sudo update-initramfs -u
sudo systemctl restart gdm3
thank you for replaying and taking time to respond to a probleme that not yours .
Oh, it is my Problem, too. I want change that, too.
The Thing is: I didn't used another Image Name. I named it login-background, too and used an .png File. So, I didn't need to change that and it could take it like it is. I made a Reboot after it but ... nothing. That's Gnome, hahaha!
A while ago, I made here a Feedback Thread for integrate the Customization of the Login Screen in Zorin. Maybe they can built that in and I will see that in the Future.
Did you:
I understand what you mean, but I would recommend doing as I typed in case there is another file referencing login-background.png. In this way, having the css call on a different file name may narrow the test.