My Microsoft Word looks like a square.

What does it look like when you select "Draft" mode?

As there are fewer Office users in Apple's forums that one would expect, you may be better served by also asking in Microsoft's user forums:

Results in Office - Microsoft Community

Typically, MS made their forums less user-friendly than before by making it harder to limit answers to Mac subjects, but it can be done when you know the filter trick:

Use the Sub-Topics pull-down to limit answers to Macs, and the Show Filters option to further narrow the results.

Everyone there is both and Office user and a Mac user, something you can't say about Apple's forums. I believe you will find the contributors there knowledgeable and very helpful. If this is something, as we suspect, that needs Microsoft's attention, the contributors there should also be able to supply you with a feedback link.

This is what Word looks like when you have selected Dark Mode. The text box doesn't turn black, it remains white like a sheet of paper. Frankly, I'm less than thrilled with Dark Mode because so many apps do unexpected things though I'm not sure if this one is unexpected or not.