That is the idea. I don't know what exactly you mean by a themed relay, but you can invite like-minded people and get them to do manual themed curation using the /favorites sub-relay.
Or you can get them to talk to each other either in groups or in the /internal relay about some theme, like a community.
Or you can get them to curate public posts submitted to the /moderated relay, ensuring those meet some standards.
If you are aiming for a curated experience you must be more selective with who you invite, as anyone invited will have the power to curate decide what goes in the thing. But if you're aiming for a community to live inside the relay you'll want to be more liberal and get more people.
Let me know if you need some crazy feature that isn't implemented yet.