Spontaneous color scheme changing. That is a problem I have not heard of.
Do you notice any pattern to the switch? If you don't, pay attention and try to identify what is happening around the time the problem happens. When you identify the situation that triggers the problem, then we have a better chance of suggesting a fix.
Is always falling back to the same one?
Does the fall back always happen after restarting the computer?
Next time you have the problem, Just for the "halibut"
try this. Restart the computer, then use "run as admin" to start an Office application, change to the desired color, then shut down the application normally and restart the computer normally (don't just hit the power switch!), See if the change "sticks".