root@ubuntu:/usr/share/lightdm/lightdm.conf.d# lightdm --test-mode -v --show-config
   [SeatDefaults]
A  greeter-wrapper=/usr/lib/lightdm/lightdm-greeter-session
B  guest-wrapper=/usr/lib/lightdm/lightdm-guest-session
F  greeter-session=lightdm-gtk-greeter
D  xserver-command=X -core
F  autologin-user=student
F  autologin-user-timeout=0
F  user-session=Lubuntu
F  bg=/usr/share/lubuntu/wallpapers/wallpaper_lodz.png

Sources:
A  /usr/share/lightdm/lightdm.conf.d/50-greeter-wrapper.conf
B  /usr/share/lightdm/lightdm.conf.d/50-guest-wrapper.conf
C  /usr/share/lightdm/lightdm.conf.d/50-unity-greeter.conf
D  /usr/share/lightdm/lightdm.conf.d/50-xserver-command.conf
E  /usr/share/lightdm/lightdm.conf.d/60-lightdm-gtk-greeter.conf
F  /etc/lightdm/lightdm.conf