Processes in Microsoft 365 for setting up Office apps, redeeming product keys, and activating licenses.
In Office 365 (2016) the problem seems to be resolved by following some advice given by someone else in this thread, that I lost track of, which is:
in Options - General - Personalize your copy of Microsoft Office
check the box for "Always use these values regardless of sign in to Office
So far this has worked and maintains the Dark Gray theme which I prefer across the Office applications suite.