Please make sure your device is on latest software version. You may check for the latest software updates by going into Device Settings > Software Updates
If your device is already on latest software version, please follow steps to enable to dark mode in your device:
realme UI 1.0: Device Settings > Display & brightness > Dark Mode : On
realme UI 2.0: Device Settings > Display & brightness > select Dark
To Enable or Disable Dark mode for installed third party applications, please follow below instructions:
realme UI 1.0: Device Settings > Display & brightness > Dark Mode : On > Dark Mode for Third Party Apps
realme UI 2.0: Device Settings > Display & brightness > select Dark > Dark Mode settings > Dark Mode for Third Party Apps