You can enable Dark Mode by opening Control Center long-pressing on the screen brightness option and then tapping Appearance at the bottom of the screen. This method is just a quick toggle to turn Dark Mode on or off, but there’s another option that automatically switches between light and dark modes.
Open the Settings app, go to Display, and at the top will be the option to switch between light and dark, or turn on Automatic. You can then set your iPhone to switch at sunrise and sunset, or set your own schedule.
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