I believe our default IntFiction theme is pretty close to the default Discourse theme (at least as it was when we started using Discourse) with a little bit of color tweaking and a moderate amount of UI configuration.
Horizon is actually growing on me for its lack of clutter - and I’m actually not mad at it in light mode even though I’m basically half-grue; and that it’s not devoid of color on the category page.
I can set Horizon as the active theme - I don’t think it will change people’s existing settings but would become default for new users.