Hi PollyWogg 
Windows themes are a combination of Preferences, a Palette, a Stylesheet and a XML-manifest, zipped in a .scrformat file.
You won’t be able to change a Theme updating File > Options > Appearance settings alone. Moreover the colors in the palette are used for several User Interface elements at the same time. The Stylesheet doesn’t cover all UI-elements, but enough of them create a good looking Theme. See Scrivener 3 Themes (Windows) for three complete themes (blue, green, brown).
Mac and Windows Themes are not compatible, as you’ve already discovered.
You can find hexadecimal color codes quickly with the windows color picker using the button Pick Screen Color. But a Theme comprises more than color codes.
The best way to start is to pick a Theme closest to the result your striving for, indeed.
Then you need to adapt that Theme file to your intended result.
Reaching a 100% pixel perfect result is probably unrealistic, but you can come a long way with some perseverance. 