The complete Dark Mode wasn't running on a mobile device, but rather an iOS 10 instance within Xcode. We found it on by default within the alarm app.
Dark mode was previously rumored for iOS 10, but wasn't announced on stage during WWDC 2016 earlier this week.
It appears the Dark Mode can be found within iBooks, Safari, Alarm, and iTunes, with a little code trickery. It doesn't appeared finished, as Game Center has incomplete menus within the Dark Mode.
Apple has previously experimented with a Dark Mode feature in the iBooks app on iOS 9 and systemwide on OS X Yosemite.
iOS 10 is Apple's next major software update for iPhone and iPad users, set to be released this fall. It adds a new user experience, open APIs for developers, Siri intelligence to the iOS keyboard, a new Home app, facial recognition within Photos, and more.