question

Hi @Angie-3825,

Since last week, my outlook keeps converting back to the default white theme whenever I set it with the Dark Grey

What's the version of your Outlook 2016(File>Office account>About Outlook), please first ensure that you have updated to the latest version of outlook client.
Considering your issue occurred only recently, have you performed any specific operations before it(such as install any programs or add-ins)?

Could your other office clients work normally with your theme(such as Excel, Word)? If it also couldn't work normally on your word and excel, how did you set your office theme(via options setting in your office client or via registry editor)? Based on my tests, if we changed the theme via UI theme register key in register editor, the setting seems to not work and your clients would convert back automatically, if it's your case, please try to change your office theme by other ways.
Also, according to my research, the issue may also occur when your Office account has any problems there, it's also suggested to sign out and re-sign in it to check if the issue has any difference(File>Office account>sign out)

If you actually change the setting via option setting in Outlook client, as I know, the office theme settings could sync to other office client in your pc, so we could try to change the theme on your Excel or Word via File>Office account>change the theme, then check if the issue has any differences on your Outlook.

If your Outlook client still couldn't display your setting in other client and the office theme work normally on your Word and Excel, the issue may be more related to your Outlook itself, in order to avoid the interference of Outlook add-ins, it's suggeted to disable any unnecessary add-ins in your Outlook(File>Options>Add-ins>COM-addin>Go>uncheck it)

Hope your issue would be resolved soon.

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