iOS Smart Invert (Settings / General / Accessibility / Display Accommondations/ Invert Colors = Smart) not only keeps the screen darker, but with the iPhone X it extends batter life by 40%+.
Unfortunately SkySafari 6 Pro (and probably others) misbehaves with Apple iOS smart invert. The screen switches to mostly white rather than staying dark - dumb rather than smart behavior. Night mode is even worse with smart.
Please make this work!
