KDE customization questions?

Is there anyway to customize the login screen, lock screen, kde loading screen beyond what's given to you?

I've read that you can replace image files of already existing themes if you want a little bit of customization; however, is this safe / will it break future updates to plasma?

For the lock screen, I know I can change the background but I'd like to change the foreground template / login form. E.g. change how transparent the foreground is...etc

Thanks in advance - as always!