The apps are installed as flatpaks, and I did allow permissions to my ~/.themes folder where I copied Zorin's themes, but they are not being applied.
I think these are good Instructions to make that. They offer the GUI Way with Flatseal and the Terminal Way:
Thank you! I actually did many things mentioned in the article, using both Terminal and Flatseal, but it now finally worked when I added a GTK_THEME environment variable. I tested out 14 Flatpak apps (all Gnome apps), and the theme successfully applied to 13 of them. For some reason it doesn't apply to the Eyedropper app. It seems Zorin themes could use an update to apply properly to new designs, such as the About dialogs of Gnome apps:
Also, seeing how the Zorin team added more theming options in Zorin Appearance (including a link for installing third-party themes), it would be beneficial if the guide included things from the article you posted here.
I don't know if this is a proper place to suggest new things, if not - somebody let me know.
To suggest Things, you can simply start a Feedback Thread and there You can explain/describe it.