Ever wanted to switch to a dark theme on iOS devices (iPhone, iPad or iPod Touch)? Partial dark mode, which dims the dock and main menu, has been available on Mac since OS X Yosemite, but Apple hasn't added an official dark mode / theme to its mobile iOS yet. Because dark mode is white text on a black background, it creates a more comfortable environment for the eyes (especially at night).
Steps
Method 1 of 2: Enable Smart Inversion (iOS 11)

Step 1. Upgrade to iOS 11 or later
Open up

Settings> General> Software update> wait while the device checks for updates> click on Download and install or Install now. Since Smart Inversion has been added to the OS since iOS 11, you need to update to this version to enable it.
Please back up your data before updating iOS. Read one of the following wikiHow articles to learn how to update your device: How to back up iPhone (and iPod Touch) data or How to back up data to iPad

Step 2. Open the Settings app

Find and tap the gray gear icon that says Settings. Typically, it can be found on the first page of the desktop.
To get to the desktop, press the Home button (the round button at the bottom of the device). Swipe left and / or right to switch between desktops until you find the application you want

Step 3. Go to the section

Basic.
Typically, this section opens by default and can be found in the left pane, but if the Settings app was already open, chances are that you will find yourself in a different section of the app.

Step 4. Go to the "Accessibility" section
Select Accessibility from the list of settings. It contains options and functions that can simplify the operation of the device.

Step 5. Go to the "Customize your display" section
Select Display Customization from the list of settings. The section "Display adaptation" includes special settings for visually impaired users.

Step 6. Touch Invert Color
This section contains both the classic color inversion settings and the new Smart Inversion feature.

Step 7. Activate the switch

"Smart inversion".
The following is the official description of how this function works: Smart Color Inversion reverses the colors of the display, except for images, videos, and some programs that use dark color styles.
Method 2 of 2: Enabling Standard Color Inversion

Step 1. Open the Settings app

Find and tap the gray gear icon that says Settings. Typically, it can be found on the first page of the desktop.
To get to the desktop, press the Home button (the round button at the bottom of the device). Swipe left and / or right to switch between desktops until you find the application you want

Step 2. Go to the section

Basic.
Typically, this section opens by default and can be found in the left pane, but if the Settings app was already open, chances are that you will find yourself in a different section of the app.

Step 3. Go to the "Accessibility" section
Select Accessibility from the list of settings. It contains options and functions that can simplify the operation of the device.

Step 4. Go to the "Customize your display" section
Select Display Customization from the list of settings. The section "Display adaptation" includes special settings for visually impaired users.

Step 5. Tap the Invert Color section (iOS 11+ only)

Step 6. Activate the function switch

Color Inversion or Classic Inversion on iOS 11+.
This will change all the colors on the screen from white to black. Since most of the iOS interface has a white / light gray background, the background will turn black.
Advice
Try changing your keyboard shortcut settings to enable Smart Invert when you triple-press the Home button. To do this, go to Settings> General> Accessibility> Keyboard shortcut and select Smart Invert
Warnings
- Smart inversion is not friendly with many third-party apps (non-Apple apps). For example, it can invert the colors of a photo in the Instagram application, but in the standard Photos application, the photos will be displayed as if the Smart Invert function is disabled.
- Enabling Smart Inversion disables Night Mode.