The Zorin group team (responsible for the development of ZorinOS) have a forum that you can consult here
Also, they have a FAQ section
that says that you can use something called Zorin Look Changer to customize your OS.
If that doesn't work for you, please check this link
that explains how to do the same thing in Ubuntu-Gnome (it should work in ZorinOS since it's an Ubuntu derivative).
Let us know if this works for you. Have a great day!