Open the LightDM Configuration File
The syntax the command to edit the LightDM configuration file is:
sudo [text editor] /etc/lightdm/lightdm.conf
To use nano (the default text-based terminal editor) enter the command:
sudo nano /etc/lightdm/lightdm.conf
Otherwise, you may find it easier to read and edit the file by using gedit (or mousepad in recent releases), which is a graphical text editor:
sudo gedit /etc/lightdm/lightdm.conf
Reboot your system to see if it works as intended.