Ubuntu – Cannot change lock screen background on Ubuntu 18.04

Before you mark the question as a duplicate, I read all of the posts regarding this, but nothing helped me. I'd like to change the background picture of the lock screen.

I changed the ubuntu.css and gdm3.css accordingly to

#lockDialogGroup {
  background: #000000 url(file:///usr/share/wallpapers/minimalistic-gray_00323985.jpg);
  background-repeat: no-repeat;
  background-size: cover;
  background-position: center;
}

Which results in a change of the Background of the log in screen, not the lock screen. When I press super + L the screen containing time has the correct background. When I swipe it up to tipe in my password, the purple Ubuntu screen still appears. If I click to change the user, the log in screen appears with the right background. I have the same background picture enabled in gnome tweaks for desktop background and lock screen.

I'm running Ubuntu 18.04 GNOME on a TuXedo Laptop. And I use Communitheme – maybe thats the problem?

Any ideas why it doesn't work? (I rebooted multiple times, so that's not the problem.)

EDIT: The output of sudo nano /usr/share/gnome-shell/modes/ubuntu.json is

{
    "parentMode": "user",
    "stylesheetName": "ubuntu.css",
    "enabledExtensions": ["ubuntu-dock@ubuntu.com", "ubuntu-appindicators@ubuntu.com"]
}