hdansin Nautilus doesn't use libadwaita (yet. It will in GNOME 43, assuming their port is done this time), just libhandy.
I tried looking into this earlier, with the help of people on Ubuntu/UbuntuBudgie and Fedora, with no results. Nautilus uses the set GTK theme as expected on those other distros, and I can't figure out why it is stuck on Adwaita on Solus. The only relevant package that was updated this cycle that might be related (to my knowledge) is libhandy, but the other distros also have the same version, and it works there. So I'm stumped so far.
The really strange thing is that (sometimes) I can get the header bar to change themes, but not the rest of the application. I don't know how that's even possible.