$ sudo apt-get update
$ sudo apt-get install gtk3-engines-unico 

gtk3-engines-unico is installed in your system.

Make ensure the gtk3-engines-unico package were installed using the commands given below,

$ sudo dpkg-query -l | grep gtk3-engines-unico *

You will get with gtk3-engines-unico package name, version, architecture and description in a table

gtk3-engines-unico package basic information: