Processes in Microsoft 365 for setting up Office apps, redeeming product keys, and activating licenses.
Check under "File" -> "Office Account" Mine was asking to update my login. That didn't work so I signed out and back in and now all acts normally.
My guess is that the office theme is account based and if office has an issue connecting while opening it reverts back to the default.
It's been a year now, this should be fixed.