Smart Invert in iOS 11 is as close to Dark Mode as Apple gets for now
Perhaps it has something to do with joining the dark side, but Apple has teased us for years about a system-wide Dark Mode and has never delivered. Perhaps the force is not strong enough to make this a feature on iOS even though every year there is a rumor that it is coming. While iOS does have an Invert Color feature, this is not exactly what the Apple faithful have been hoping for. The latter feature will invert colors used on photos, emoji and Message attachments.
Those using a beta version of iOS 11 on their iPhone can check out Smart Invert by following these directions. Go to Settings > General > Accessibility > Display Accommodations > Invert Colors > Smart Invert. And since a public beta version of iOS 11 has been released, you don't have to be a developer to check it out.
source: TechAssist via BGR
Things that are NOT allowed: