@vegelund Taskbar is black instead of white. It may be the default and the reason why you didn't notice.
Or, you may have been using the custom option and Opera couldn't infer the theme because of that.
Examples with my PC and the settings app and Opera:
Light mode
8bfe8b6a-117b-4c67-ba6c-a12bf3a3396b-image.png
Dark mode:
ea7e26cb-f79e-4807-8e01-7cc0135b53c0-image.png