How to activate Dark Mode in iOS 11

iOS 11's new "Smart Invert Colors" feature is a dark mode, but smarter. Here's how to turn it on.
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How to Unlock Apple's 'Dark Mode' in iOS 11, 12 & 13 for iPhone « iOS & iPhone :: Gadget Hacks

Implement In-App Dark Mode Using Swift Observation Protocols, by Lee Kah Seng

Request] [Tweak] Enable Smart Invert Colors Only on LockScreen - Creates Perfect Dark Mode in iOS 11 : r/jailbreak

How to enable Dark Mode on iPhone and iPad

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Check out iOS 11's hidden dark mode - CNET