The visual refresh (= the new look of Office) was introduced gradually, starting a couple of years ago. See https://support.microsoft.com/en-au/office/the-new-look-of-office-a6cdf19a-b2bd-4be1-9515-d74a37aa59bf . You can turn it off if you see the megaphone icon on the title bar, but many users report that they can no longer see that icon, meaning that there is no way to avoid the new interface anymore.
You can send feedback to Microsoft using the Feedback facility.