Well you've made me go take a look now, not something I've bothered to look at, but this is a start:
I unfortunately have to get ready for a wedding, but the file to edit the main screen of zorin-appearance the way you want is
/usr/lib/python3/dist-packages/zorin_appearance
That being said, as an example I have added the "zorinMagenta" to both themes and theme backgrounds, however I haven't gone any further as to locate the next location to make it function correctly.
EDIT: Make these changes at your own peril. I love breaking things, so I don't care what happens, but yeah