The most important reason that the themes are bad are for medical reasons.
They are designed by youngish American males with good eyesight.
Those of us getting older, or who have other eye problems, have difficulty using these themes. Glare and difficulty differentiating items on the screen makes for stress.
So we need to get official documentation saying that these themes are harmful to eyes. That will get Office disqualified from many governments and large companies. Without that, we won't get any change. They listen to money.
Second item, is both cultural and personal preference. Color choice affects mood. To many people and cultures, black and gray mean death, and are rather gloomy. Obviously, some like that choice, but many don't.
Microsoft and Windows used to be for customization for the user. It's lost that. It's forcing the taste of a few people on the masses, that's extremely arrogant.
-RickH