In order to style the background and theme of the LightDM login screen edit the /etc/lightdm/lightdm-gtk-greeter.conf file as follows

#nano /etc/lightdm/lightdm-gtk-greeter.conf

[greeter]
background=/home/oscar/Pictures/Katwijk-Silouet-edge-grain1.jpg
theme-name=Adwaita-dark

In a VM the lightDM could start with a wrong/low resolution. Add the following line to /etc/lightdm/lightdm.conf:

# nano /etc/lightdm/lightdm.conf

[Seat:*]
display-setup-script=xrandr -s 1920x1080
#type=xlocal