With iOS 1
How Dark Mode Works
Dark mode is performed using a toggle switch in the Control Center. You can turn it on or off at any time. All standard Apple apps and the system interface support the dark theme.
Apple has chosen a real dark mode. This means that the background is jet black in most places in the user interface. On iPhones with OLED screens – that's the iPhone X, XS and XS Max – this has a twofold effect.
At first the white text looks sharp on the pitch-black screen. Second, since the black background does not light up the pixels, you will end up saving a bit of battery life. iPhones and iPads with LCD displays – all other iPhones except X, XS and XS Max, including iPhone XR and iPhone X – show no difference in battery life. Since most of the Internet usage is focused on white background websites and color photos, most will not save too much energy.
Apple has also added a series of dynamic background images that automatically change depending on the dark and light modes. You can find them in the "Still Images" section of the "Background Image Selection" screen in the Settings app.
CONNECTED: Why I want an iPhone on iOS 13
Enabling Dark Mode  If you set up your iPhone or iPad after upgrading to iOS 13 or iPadOS 13, it will be a new one Appearance screen is displayed. Here you can select the theme "Dark" to activate the dark mode from the beginning.
You can also switch between the dark and light modes at any time through the Control Center or App Settings. To change via the Control Center, tap and hold or 3D Touch / Haptic Touch on the Brightness slider.
Tap there on "Presentation" button.
To enable Dark Mode from the Settings app, go to the "Display and Brightness" section. Here you see the option "Appearance" at the top of the menu.
Below is an option "Automatic". After activation, you can set the dark mode to turn on automatically at sunset or during a custom period of time. Tap the Options button to adjust the schedule.
How Dark Mode Works with Sites and Apps
Just like macOS Mojave, Dark Mode is universal in iOS 13 and iPadOS 13. If a website supports CSS Dark mode, Safari will automatically download the Dark Theme version for you (as you can see in the screenshots below).
This feature is enabled by default. To turn it off, you can turn off Dark Mode CSS support in Settings> Safari> Advanced> Experimental Features.
Same goes for apps upgraded for iOS 13 and iPadOS 13. When you flip the switch, apps automatically switch to dark mode. Once iOS 13 is released in the fall of 2019, we'll have more details on how this works and whether it's possible to override this system feature, forcing a particular app to always open in bright mode.
Dark Mode Interface in iOS 13 and iPadOS 13
Apple has changed some important UI elements to work in dark mode. Although it's not exactly skeuomorphic, Apple brought back some of the shadow and texture elements from the old iOS 6 days.
For example, when you open a panel in an app, such as App Store or Music, it will be displayed over the current screen. You can still see the lowest level where it is not hidden.
When you open the new Sharing control panel in Safari, Apple hides the Web page. When it comes to tabs, Apple now shows the difference between the selected and unselected tabs based on depth. And for the sake of readability, Apple uses a true black color just for the background.
Overall, most UI interactive elements are actually a shade of gray. It's this grayscale you'll find throughout the system's user interface, including the background of notifications, widgets, and the control center.
Dark Mode in Apple's Stock Apps
Apple's stock apps serve as inspiration and backdrop for third-party developers. If we look at how Apple has implemented the dark mode in its own apps, we can guess how third-party apps will adjust the dark mode in the future.
The News app is a great example of how this works fine, the dark mode may look good when it's implemented. The real black background paired with the blue accent color in messages works very well.
Welcome to the Dark Side
Dark Mode for iOS 13 and iPadOS 13 is a collection of design changes and optimizations that have taken years to complete to perfect. What started with the Watch app has now affected the entire operating system.
With dark mode soon to be available on all major Apple operating systems, it will be interesting to see how Apple is improving and changing the design in the future.
Apple's iOS 13 and iPadOS 13 are currently available in beta and will be available in the fall of 2019 as a stable operating system update.