iOS 11 introduces a display setting that inverts some but not all of the colors on your device. iOS has long had a setting to invert the colors on your device, but now it has a smarter invert option that leaves photos, videos and other elements alone. It's the closest thing you'll find to a dark mode for your iPhone or iPad.
Head to Settings > General > Accessibility > Display Accommodations > Invert Colors and toggle on Smart Invert. (You'll notice that the old invert option is now labeled Classic Invert.)
With Smart Invert enabled, you'll immediately see that the background for Settings app turns black but the toggle switch background color remains green instead of turning purple like it does with Classic Invert. The app icons also remain unchanged. Open the Photos app and you'll see a similar effect -- the background is black but the photos are left alone so they don't look like photo negatives. Also, Smart Invert is smart enough to keep its hands off an app with a dark background, such as the Clock app. Meanwhile, on your home screen, your device's wallpaper doesn't get inverted but the dock at the bottom does and the white text at the top turns black.
What do the aforementioned apps have in common? Yep, they're all from Apple. Smart Invert is less smart with third-party apps. Instagram and YouTube, for example, are rendered useless with Smart Invert because the colors for photos and videos on each app are inverted and look crazy. Same for news apps such as Huffington Post, The New York Times and Newser, but Apple's News app looks just fine with Smart Invert. Like the Clock app, Spotify has a black background, but because it's not made by Apple, its background turns white and album art exhibits insane, inverted colors with Smart Invert.
Perhaps Smart Invert will get smarter when the final version of iOS 11 is rolled out this fall, but for now it's best used sparingly. You aren't likely going to drill down into the Settings app to enable it for use with a particular app, only to then head back to Settings to turn it off again. Thankfully, you can set an Accessibility shortcut for Smart Invert that will let you triple-click the Home button to toggle it on and off. To set up the shortcut, go to Settings > General > Accessibility, scroll all the way to the bottom to Accessibility Shortcut and select Smart Invert Colors. That's the smartest way to use Smart Invert for my money.