The Stranger Things mode is a cool and fun feature Spotify added to promote the release of season 2. It changes the appearance of the mobile app and desktop app while listening to the series soundtracks to match what the world of the Upside Down looks like.
It will only work on the show soundtracks though, but an idea has been added to have Spotify create more fun features like this in the future. You can vote on it if you would like to see overlays and designs for any playlists! 🙂