The debut of the iOS 13 and iPadOS betas brought the much-desired system-wide Dark Mode option. Dark Mode is designed to be easier on the eyes in situations with low ambient lighting, yet maintain contrast.
OLED iPhones, including the iPhone X, iPhone XS, and iPhone XS Max, may also prove to extend battery life whenever true blacks are on the screen, since black pixels on an OLED screen are for all intents and purposes turned off, which will theoretically save power.
You’ll also find that the Display & Brightness settings screen also includes an Automatic switch that you can be toggled This will cause your device to automatically enable Dark Mode at sunset, and go back to Light Mode at sunrise.
A custom schedule can also be set to control the display change in the Options menu that will appear when the Automatic switch toggled to On.