If iOS 11 is too white for you, it’s easy to make it darker: go into (deep breath…) Settings > General > Accessibility > Display Accommodations > Invert Colors and switch on Smart Invert.
Unlike a normal invert, which reverses the colors of everything on screen, Smart Invert doesn’t mess with images and other media: it’s effectively a Dark Mode for iOS. The image shows you the difference. The left image is regular bright white, the right image is Smart Invert.
I tried it and it’s great.