Hi Jakabcin,
Normally, we can change the look and feel of all your Office programs by changing the Office Theme.
To change the background color and design of all your Office programs, go to File > Options > General > Office Theme.

If you want to customize themes, I’d like to recommend you visit the Outlook user voice forum:
https://outlook.uservoice.com/ to post your ideas.
It is a place for customers to provide feedback about Outlook. What’s more, if an idea is highly voted there by other customers, Microsoft Product Team will take it into consideration when designing the next version in the future.
Thank you for your understanding and support!
Regards,
Perry
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