Yes, I use and recommend @Storm 's method here:
All necessary libraries to use themes are already installed and come with Zorin OS.
If a user installs a Higher version of Python than their system supports, then many applications that rely on Python won't be able to "see" Python to access and use it. There really are very few instances that can possibly require you to replace or upgrade Python.
It does not matter for icons. On Icon sets, you need to get to some pretty old sets before you see any problems and even those can usually be easily corrected by adjusting the index.theme file.
GTK version matters within ranges- I will expand on this in a moment...
Gnome.
Gnome is opposed to user theming and is very vocal about this. In the changeover to gtk3, Gnome deliberately broke theming in order to put an end to it.
By changing the .css with every incremental release of gtk3, they caused themers to have to constantly find and adjust themes for the changes Gnome made. Horst, KillHelloKitty, JamesHardy88, TechnoShaun and a large number of "old days" themers vanished. K.H.K. and Horst knew .css better and stuck around longer... But in the end, they got tired of having to Fix The Breakage between each release.
With the release of gtk 3.20 - The Stakeholders for Gnome made Gnome devs sign a pledge to Stop breaking themes. Though they moderately still break that promise in small ways, over-all any gtk theme from 3.20 up should work... But will not work on gtk4.
Looking up Gnome Themes, you will see older gtk 3.16 or 3.18 themes... These will not Work on Zorin OS 15 or 16.
Stick to 3.20 and up (Not including gtk4) and you will be fine.
GTK4 currently breaks all themes except themes made (and locked in) by Gnome.