Hi,
What's the detail version of your Outlook? (File > Offcie Account > About Outlook)
In this situation, please go to File > Office Account > Office Theme, make sure the Colorful is selected.

If this is not the case, please provide me with the screenshoots of your "Folder Pane" and "Control Ribbon" so I can do further research. (If you cannot put the screenshot here, you can upload it to OneDrive (https://onedrive.live.com/)
and then share the link here.)
Hope this can be helpful.
Regards,
Aidan Wang
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