Not able to change the login screen background ubuntu 20.04

There is no simple way to change the background on your login screen in Ubuntu 20.04. The people who make Gnome are pretty insistent about this¹, it seems. If you really want to modify it, though, you'll need to build a modified focalgdm3.gresource.xml file and force the change through.

And this is how you do it:

0. In Terminal, make sure you have the git and libglib2.0-dev packages installed:

   sudo apt install git libglib2.0-dev
   

1. Clone Pratap Kumar's Focal GDM3 login screen hack:

   git clone https://github.com/PRATAP-KUMAR/focalgdm3.git
   

   ^{Note: This tool is designed for 20.04 specifically. It will not work on any other version of the system.}
2. Enter the focalgdm3 directory:

   cd focalgdm3
   

3. Run the cloned script as root, understanding that you're running code from a GitHub repo with full permissions. You have two options:
   1. Set a background wallpaper:

      sudo ./focalgdm3 /home/yusof/myloginbackground.jpg
      

      ^{Note: Be sure to use the full path. Do not use ~.}
   2. Set a solid background colour:

      sudo ./focalgdm3 \#FFFF00
      

      ^{Note: You may not want to use #FFFF00 as it's bright yellow.}
4. Restart the computer to see the changes. Signing out is not enough to trigger GDM3 to reload the necessary .xml file.

If at any time you would like to return Ubuntu to the stock background, you can reset the login screen like this:

sudo ./focalgdm3 --reset

¹ See "The Options Question" down near the bottom of the blog post if you're interested