There are several ways to enable the Dark Mode. You can head over to settings then Display and Brightness and toggle the Dark Mode or you can just pull down / up the control centre, long press the brightness slider and toggle it from there.
Another way is to automatically enable the Dark Mode, you can select the option to enable it from sunset to sunrise or you can also set time when you want this feature to enable and disable itself.
Let’s take you to the hands on experience with iOS 13 Dark Mode. Some apps like weather are still the same, it seems that Apple will work on these apps in the future beta updates. Take a look.
Contacts and Dialer
Screen shots courtesy: 9to5mac