$ sudo apt-get update
$ sudo apt-get install razorqt-lightdm-greeter 

razorqt-lightdm-greeter is installed in your system.

Make ensure the razorqt-lightdm-greeter package were installed using the commands given below,

$ sudo dpkg-query -l | grep razorqt-lightdm-greeter *

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

razorqt-lightdm-greeter package basic information: