The dark design, also known as Dark Mode, does not find its own option in the OnePlus smartphone's settings, but is changed via the system tone, which adjusts the layout of the system.

The dark theme or design changes all system layouts and elements of the home screen, settings and compatible apps.

The function is not only easier for the eyes in low light conditions, but also saves battery power.

Android 10 | OxygenOS 10.0
Step 1:  Open the  Settings
Step 2:  Tap on  Customization
Step 3:  Tap on  Tone
Step 4:  Choose  Dark
Step 5:  Tap on  Save
  1. Open the Settings
  2. Tap on Customization
  3. Tap on Tone
  4. Choose Dark
  5. Tap on Save

OnePlus Instructions