Replace the themes directory with the "layouts" directory which contains all the templates. I do my blog this way. https://github.com/PowerSnail/PowerSnail.github.io

OK I see, but even though it's themeless you still have to follow the Hugo theming structure (_default, partials, index.html, etc). Mine is the same as that, it's just nested into the themes directory.

It's that theming structure that was a pain to figure out the first time for me

Yeah, that template look-up hierarchy is unavoidable. The benefit of being themeless is mostly organizational: one config file, one asset directory, etc.