iOS 14 (and iOS 15, I’m sure) 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.
(This how-to is based on my experiences and info on Apple’s support pages — where the images sometimes come from.)