Hi Rebecca,
Actually you can pick any of the default backgrounds and/or themes, but you cannot customize the look or the colours specifically. Please feel free to check out this article for more information:
https://support.microsoft.com/en-us/office/change-the-look-and-feel-of-office-63e65e1c-08d4-4dea-820e-335f54672310
Regards,
Burak V.