Glad to hear this issue has been resolved! Thanks for the sharing.
However, due to a recent update in forum policy, the question author now is not able to accept their own answers.
So, I have written a brief summary of the solution this issue. Please feel free to accept it as the answer, which would benefit others who also has similar issues in forum.
[Why is my “Show apps in Outlook”-menubar white?]
Issue: The status bar on the left side of the colorful Office Theme from the Outlook client settings turns white.
Solution: The problem was resolved after a full reinstall using the Office uninstall tool.
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