Hello Horst, this is Emily.
There can be several possibilities.
I assume you were referring to your entire Windows being in dark theme? If so
Go to Settings > Personalization > Colors.
For Choose your color, select Light. See if this helps.
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If you are just saying your web browser is now in dark theme, then depends on what browser you are using, if you are using Edge, put this address in the address bar: edge://settings/appearance then select the light theme.
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If neither works, please feel free to share a screenshot so I have a better understanding of what is black and white so I can guide you to the right place.