A new test build of OmniWeb is now available, based on WebKit 2 and with support for Dark Mode. It definitely has plenty of known rough edges (and probably plenty of unknown as well!), but feel free to give it a whirl if you’re feeling brave!
(I’m still using OmniWeb as my primary browser, though until I got password autofill working again I ended up launching Safari a lot as well just to log into sites that required authentication. I’m glad that’s finally sorted out in this build!)