> You're using the base16 theme for the first one, correct?
Yup.
> I liked the second one as well. What theme is that? [...] Also, you've mentioned in the sidebar that you've built it using Grav and Hugo.
The theme is a Grav theme, ported to Hugo[1] (hence the "made with Grav and Hugo" -- the actual site is made with just Hugo). The source for the site is [2]. And the source for my other sites are here[3,4].
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.