I think this is nothing more than a revised Do Not Disturb mode.
There's no way Apple would promote people using their cell phones in a movie theater. Why would any professional company openly support the douchbag losers who use their phones in the most rude way possible, being on it during a movie in a public theater. I think you should be immediately kicked out of the theater if caught using it during a movie.
A true dark mode, would be considered a "theme" imo, and Apple has not released any theming on iOS yet, that would be a big deal for them, and they wouldn't just casually introduce such a huge thing for iOS in the middle of iOS 10. Trust me, they'd milk that as a major feature and be a part of the keynote speech for iOS 11, if themes ever come out to iOS