This brief tutorial shows how to easily change the login screen background wallpaper or solid color on your Ubuntu 20.04 Gnome desktop.
Editing content in Ubuntu 18.04 and Ubuntu 19.10 was fairly easy, but unless you hack the stylesheet file, there is no configuration option to change the background of the GDM3 login screen.
On Ubuntu 20.04 you need to extract the CSS file for the current theme
.gresource Edit the file, then the content and recompile
.gresource The file, and finally set it as the default.
Thankfully, Github has some scripts to make things easier.
1.) First open a terminal (Ctrl + Alt + T) and run the command to download the script.
You can also download github web page script..
2.) Run the script using the command.
sudo /bin/bash focalgdm3.sh --set
Enter the user password for the sudo prompt, press Enter to continue the script, then type 1 and then the path to the image, or 2 and then the color code.
Note: At the moment the filename of the image file seems to have spaces in it, but it is not supported.
When all is done, restart your computer for the first time to apply the changes.
3. (Optional) To restore your changes, run the following command:
sudo /bin/bash focalgdm3.sh --reset
In addition, another script is available github Do the same job.
Via: Ask Buntu