There’s something I don’t understand, maybe someone can help: where does Ubuntu MATE set the options for the default Slick greeter appearance? I’ve checked everything in /etc/lightdm, /usr/share/lightdm and /usr/share/slick-greeter and haven’t found any config file with the background used (/usr/share/backgrounds/ubuntu-mate-common/Green-Wall-Logo.png).
I’ve tried to tweak the picture and it was reflected in the login screen, which confirms that it uses this file and not a copy from somewhere else, so it has to be referenced in a config file used by LightDM/Slick but I can’t figure which one.
EDIT: just found it in another thread: it’s stored in dconf (x.dm.slick-greeter).