How Do I Turn On Dark Mode For My iPhone?
As of iOS 12 Apple has still not released official support for Dark Mode. Dark Mode can be especially helpful for reducing eye strain and making your iPhone or iPad easier to view in the dark. Until the feature is introduced, there is a setting that will serve as a useful proxy for a true iOS Dark Mode.
Launch the Settings app and go to General > Accessibility > Display Accommodations. Tap the Invert Colors option then toggle Smart Invert into the on position. This will reverse the colors of the display without reversing the colors of photos or other media. It's not a perfect substitute and it may turn some dark apps light, but it can be useful under certain conditions.