I have no idea (someone else did the port), but after looking at the two theme sources[1,2] I'm pretty sure it's non-trivial but not nearly as bad as writing a new one. They both have the same concepts (partials, templates) but the languages are quite different.
[1]: https://github.com/matcornic/hugo-theme-learn [2]: https://github.com/getgrav/grav-learn