Office 2016 Keeps Changing Office Theme

I have 2 copies of Office 2021 on my desktop PC and Dell laptop and Office 2016 on my HP laptop. The Office theme is set to Dark Grey on all PCs but for some reason the Office 2016 theme on my HP laptop keeps setting itself to the system theme which is the dark theme when I restart the PC. How to fix this?

If no one can help on this forum (I can't) I can recommend a forum that specialises in MS Office. They are very knowledgable: -

I've never had this problem, but based on what I see on the Web, it seems to be related to your Microsoft/Outlook account settings. This may not apply to 2016 or your environment.

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"In Outlook, click on File | Office Account and look for an 'Account Error' with a 'Fix me' button."

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"Changing the settings on the Web based Outlook e-mail account may stop the overriding of the local MS client"