Like Jason mentioned, there are bunch of Hugo theme directories where one can go look for a theme that could be used in micro.blog, too. One such place is https://themes.gohugo.io/. Those don’t work out of the box with micro.blog, but it isn’t a huge ordeal to make them work with MB.
I don’t think we’re going to see any commercial micro.blog theme stores in the near future, but I could be wrong!