From my understanding, there is no centralized theme manager for Gnome. You can easily change the theme up in the Zorin appearances app or alternatively you can install Gnome tweaks which lets you do it with a bit more control over it. There are a lot of themes available in the Softwares app. You can search for them there or alternatively download them from other sources on the internet. There are some websites that act as a central hub for Gnome themes and customization but I hope someone else can mention the noteworthy ones as I am not familar with them.