This no longer has any affect on the Office UI Theme... (if fully updated) The registry setting does no good, whether write protected or not. I was able to update office on one of my PCs (windows 7) to an earlier version, and the black theme is an option. On another pc, Windows 10 , rolling back the version, caused office apps to not open, had to do an online repair, which of course killed the possibility of getting the black theme. Microsoft is just as bad as ... well never mind. On a third PC, windows 10, office 16 version 1907 was able to keep the black theme with the registry setting ... hit and miss at best... It would seem to me there is a file somewhere that can be swapped out... I refuse to give microsoft any more money especially for office...