Forgive my ignorance but what is the difference between 4.1 and 5.1.2?
Because 4.1 already ported to WB
If you want to port it yourself, ask the author for resources and if you want to release it in the future ask for permission too. I think Max will be cooperative with that. There is a tool called SkinStudio from Stardock you gonna need this i think.
For a start you can read tutorial i've found:
Tutorial for 0 pixel borders
And you can read articles @ Wincustomize
And of course you will need some screenshots of the theme you requested. You can ask to someone who has a Mac.
Hope these helps