$ sudo apt-get update
$ sudo apt-get install mythbuntu-lightdm-theme 

mythbuntu-lightdm-theme is installed in your system.

Make ensure the mythbuntu-lightdm-theme package were installed using the commands given below,

$ sudo dpkg-query -l | grep mythbuntu-lightdm-theme *

You will get with mythbuntu-lightdm-theme package name, version, architecture and description in a table

mythbuntu-lightdm-theme package basic information: