Glad you figured it out. I agree that the documentation is tricky at first; I struggled with the @attribute thing for a bit. But now it makes distinguishing attributes and elements of the xml much easier.
By the way, just to clarify on thing, if you didn't set @provide-dark-mode="no", then there would be a toggle to switch between modes. The toggle persists in local storage, and is initially set to match the user's browser preferences. So if someone usually picks light mode and has that set up in their browser, it should default to light mode.