Hello! My name is Nazia.
According to your description above, it appears that you have already taken the necessary steps to change the theme described in this article:
https://support.microsoft.com/en-us/office/appl....
Unfortunately, this is the procedure to change the outlook themes if the answer is yes, and so I suggest you to provide comments and suggestions through the Microsoft feedback portal, which will alert the engineers to the problem and allow them to include it into future updates:
https://feedbackportal.microsoft.com/feedback/f...
Regards,
Nazia
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