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'?
As of Ubuntu Trusy 14.04, lightdm-set-defaults is no longer available. This is because lightdm now uses a configuration directory /etc/lightdm/lightdm.conf.d/ rather than a single configuration file. The files in this directory can be edited by hand, new files can be added, or files can be removed. There is no longer any need for a command that edits the single configuration file.
As you can see by searching the packages.ubuntu.com website, it's a command, provided by the lightdm package, which you already have I assume.
So, just run in a terminal:
sudo lightdm-set-defaults
The command is not in the path so you need to run it using the full name, i.e.:
/usr/lib/lightdm/lightdm-set-defaults
The Wiki page is incorrect - you can edit the configuration if you want, the purpose of lightdm-set-defaults is so scripts can easily modify the configuration.
