iOS/iPadOS 14/15offers 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.
You can also access Dark Mode from Control Center. Open Control Center, then tap the Dark Mode button to turn it on or off. Learn how to use Control Center on your iPhone or iPod touch, or on your iPad.
To turn Dark Mode on automatically at sunset or at a specific time, go to Settings > Display & Brightness and select Automatic. Then tap Options to set a schedule for Dark Mode.
(This how-to is based on my experiences and info on Apple’s support pages — where the images sometimes come from.)