When you tap on Dark Mode, it will open a page with three options: On, Off, Same as device setting. Tap on On and the entire app goes into Dark Mode immediately. Menus that have black text on a white background will show a black background with white text which isn’t as likely to burn a hole in your retina as that intense white background is.
Tap on Off and Dark Mode will not turn on at all. The last option is Same as device setting. This means that the app will follow the settings you have chosen for your iPhone. Using Dark Mode reduces the strain on your eyes, makes the screen easier to read, and if you have an AMOLED display, it can save battery life.