Xfce FAQ ​

1. Why Is the Background Color of the LightDM Login Page Black ​

The login page is black because background is not set in the default configuration file /etc/lightdm/lightdm-gtk-greeter.conf of lightdm-gtk.

Set background=/usr/share/backgrounds/xfce/xfce-blue.jpg in the greeter section at the end of the configuration file, and then run the systemctl restart lightdm command.