iOS 13 offers a Dark Mode that, in Apple’s words, “delivers a dramatic dark color scheme that looks great across the system and is easier on the eyes in low-light environments.” To enable it:
Launch the Settings app on your iPhone.
Tap Display & Brightness.
Under Appearance, tap Dark to switch to Dark Mode.
The Display & Brightness settings screen also includes an Automatic switch that you can toggle – doing so will cause your device to automatically enable Dark Mode at sunset, and Light Mode at sunrise.
Also, you can set a custom schedule for the display change by using the Options menu that appears when you toggle on the Automatic switch.