We currently use the "Read the docs" theme. It is a common very often used theme for documentation compiled with Sphinx.
But it might be too focused on technical documentation and we might either adapt this theme, change to a different one or write our own one.
The goal would be to have a more "design-focued" theme, which in particular itself respects the HIG it promotes.
Examples for the designs of other important software HIGs:
Material Design
iOS
GNOME
UWP apps