A family of Microsoft word processing software products for creating web, email, and print documents.
Dear Heiko,
Thanks for posting back.
Sorry for providing the wrong conclusion in my first reply. I have checked all settings again and found the related setting that will cause this situation on your side. To turn off the dark mode in Word for Web, please open any Word documents in the browser, click the View tab, click Dark Mode to turn off it. The dark mode button I want to double confirm in my first reply is available in Office Portal (go to https://www.office.com/launch/word, don't open any documents, click the Settings button).
@Jim
You are right, there are indeed relevant settings in Word for Web to turn on/off the dark mode. At first, I haven't found this setting, no official documentation mentions this feature, and some old threads and suggestions in Microsoft Feedback Portal also mention the Dark Mode is not available in Word for Web. These made me think that this feature doesn't exist in Word for the Web unless you have some 3rd party extension installed in your browser, and think the situation may related to the Dark Mode in Office Portal. Again, thanks for pointing out my mistake.
Appreciate your understanding and patience.
Sincerely,
George | Microsoft Community Moderator