The phone app on the OnePlus smartphone will be displayed in the default system theme, but can be set as light or dark default design if desired.

The advantages of the dark theme are that navigation is easier on the eyes and a reduction on the device's battery consumption.

Android 10 | OxygenOS 10.0
Step 1:  Open the  Phone app
Step 2:  Open the  menu
Step 3:  Open the  Settings
Step 4:  Tap on  Display options
Step 5:  Tap on  Choose theme
Step 6:  Choose  Light ,  Dark  or  System default
  1. Open the Phone app
  2. Open the menu
  3. Open the Settings
  4. Tap on Display options
  5. Tap on Choose theme
  6. Choose Light, Dark or System default

OnePlus Instructions