If you have iOS 13 on your iPhone (at the time of writing, it is available as a public beta; it will be officially released in September 2019), you can activate a system-wide dark mode very easily. This gives all pre-installed apps, and third-party apps that have built-in compatibility, a black or dark gray background that is more relaxing to read at night.
To activate dark mode, open the Settings app and tap Display & brightness. At the top of the next screen, you will see the Light and Dark options side by side; touch the one you want to use. You can also set the dark mode to activate automatically at certain times, such as from dusk to dawn.
If you don’t have iOS 13 yet, there are still some solutions. You can try Invert Colors mode, Low light mode, or Night shift; each of these offers some of the benefits of dark mode. Read more in our article dedicated to How to use dark mode on iPhone.