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

gtk3-engines-xfce is installed in your system.

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

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

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

gtk3-engines-xfce package basic information: