Dark Mode Settings
You can enable Dark Mode by opening the Control Center by long pressing on the Screen Brightness option and then tapping Display at the bottom of the screen. This method is just a quick change to turn dark mode on or off. However, there is another option that automatically toggles between the light and dark modes.
Open the app Settings go to Display and above you can switch between light and dark or activate Auto . You can then set your iPhone to switch at sunrise and sunset, or set your own schedule.