Look at https://forum.getkirby.com/t/customize-brand-the-panel-login-form/2294, which is for Kirby 2.x, but on Kirby 3 it is the same css technique with the exception of the option in the config.php file. Add the background picture with css too.
Look at https://forum.getkirby.com/t/customize-brand-the-panel-login-form/2294, which is for Kirby 2.x, but on Kirby 3 it is the same css technique with the exception of the option in the config.php file. Add the background picture with css too.