So, how did I get my Zorin OS 15.3 to look like this?
Before you start looking at themes, you first need to get Tweaks installed.
sudo apt install gnome-tweaks
My set up is based on a Theme for applications called MacOS-Catalina-1.0
The icons are called Cupertino-Catalina
and the Shell is McOS11-shell
When all downloaded, your Tweaks should look like this when set up correctly.
Hope this helps.