Welcome to the world of Word documentation. There is a lot that is not documented.
You can also look at Faithe Wempen's writing Understanding Themes (and Style Sets) or the Microsoft Blog introducing them: Changing your style in the new Word - Microsoft 365 Blog

text and background color are descriptive names for how the application designers expected these colors to be used. I do not know that they have inherent meanings. The simplest thing to do is create a document using these theme colors and then change to themes that have different colors used and see what it does. You are welcome to report back here with your results. My page has a document that you could download to make these tests.