You have to create a configuration file:
/etc/lightdm/slick-greeter.conf
I made the one below, specifying another existing background, theme, and icon-theme to keep it simple.
[Greeter]
background=/usr/share/backgrounds/ubuntu-mate-common/Grey-Wall-Logo.png
theme-name=Ambiant-MATE-Dark
icon-theme-name=Ambiant-MATE
draw-grid=false
Probably more is possible.