I like the Meta Branded style, I noted that we share some of their overall design (headings in Inter, violet palette as the primary color) from before they implemented it.
In the forum I’m about to re-launch we use a custom Horizon, and we are about to launch our branded version as well, with a fusion of retro/old-school and comic-style brutalist elements. Subtle, along with the unification of several components that we will integrate into our main theme.
I also like Foundation, but I got used to modern interfaces, and I no longer feel comfortable with such simple styles. I am also thinking about keeping a classic view with categories like forums used to be, ultra-lightweight and almost without images. For now, it’s on the backlog; everything is moving very fast, and priorities have to be set.