ubuntu wiki LightDM states:
LighDM configuration is governed by the lightdm.conf file, however it's not suppose to be directly edited, instead use:
lightdm-set-defaults
$ lightdm-set-defaults
lightdm-set-defaults: command not found
$ sudo lightdm-set-defaults
sudo: lightdm-set-defaults: command not found
$ lightdm -v
lightdm 1.4.0
synaptic 0.75.12 'Quick filter' finds no matches on 'lightdm-set-defaults'.
How do I install 'lightdm-set-defaults'?
lightdm-set-defaults
is probably not on your PATH Try: sudo /usr/lib/lightdm/lightdm-set-defaults