It seems to me that there used to be a tool to change the login screen somewhere in the preferences, and then, it was removed, and since at least the 21.04 release, if you want to change the login screen, you have to manually edit config files as described in this thread: https://ubuntu-mate.community/t/how-to-change-login-screen-background-in-ubuntu-mate-21-04/24384
Are things supposed to remain like this permanently?