I just installed Ubuntu 18.04 and I wanted to change lock/login screen background. I have 2 screens with different resolution and I want to have the same picture on both of them. I changed regular wallpaper and lock screen via system settings, But when I lock the computer and the I want to unlock it, the login screen has just default purple color as background. I tried to set it up by changing /usr/share/gnome-shell/theme/ubuntu.css, but I don't know which parameters and values to use, because whatever I try, the result is messed up every time (screen) - wallpaper is being cut randomly etc.
I have lockDialogGroup set like this right now
#lockDialogGroup {
background: #2c001e
url(file:///home/user/Pictures/Wallpapers/wallhaven-157848.jpg);
background-repeat: repeat;
background-size: cover;
background-position: center;
}