For what it's worth, what worked for me here at work was that I signed out of my Office account completely (and remained signed out) then changed the Office Theme.
In any Office application, go to File - Account (or Office Account). Once signed out, I changed the Office Theme to what I wanted the theme to be, Dark Gray. I have not had any issues in it changing back since.
What I figure must have been going on was that I was signed in to my Office Account on two different computers. Once I opened an Office application at home, the setting from there was captured in my account and the next time at work opening an Office application
it would apply that different theme. Thus if I had Colorful theme set at home and was signed in, then came to work, it would remember that last setting and apply Colorful here at work.
Hope this helps someone else...