It's FOSS - https://itsfoss.com/install-themes-ubuntu/
Brief: This beginner’s guide shows you how to install themes in Ubuntu 17.10. The tutorial covers installation of icon themes, GTK themes and GNOME Shell themes.
Newly released Ubuntu 17.10 looks good but it can be tweaked to look better. One of the reasons why I like using Linux is the flexibility of customization. Changing themes gives the system an entirely new look and feel. And the best thing is that there are tons of good themes for Ubuntu and other Linux at your disposal. You can play with them as you like.
In this tutorial, I’ll show you various type