Go ahead, pull an iPhone out of your pocket right now, press and hold the Home button and tell Siri to please “turn on dark mode”.
Much to your surprise, instead of responding with a “I didn't quite get that”, “I don't understand” or some such, she'll inform you that “I'm not able to change that setting”.
As noted by AppleInsider, the wording is unusual given that iOS currently contains no reference to any kind of dark mode. Is Apple trolling us ahead of the keynote?