Go to Settings > Display & Brightness.
Select Dark to turn on Dark Mode.
To turn Dark Mode on or off from Control Center, pull down from the top-right corner of your device, then touch and hold the brightness control . Then tap Dark Mode On or Dark Mode Off.
You can also set Dark Mode to turn on automatically at sunset or at a specific time. Go to Settings > Display & Brightness and select Automatic. Tap Options to set a schedule for Dark Mode.
iPhone supports dark mode. Learn more about iPhone on the website.