I’m sorry, I’m not aware of a way to do this currently. You can remove the color selector, so that users can’t choose custom colors. It’s not the same as forcing a specific pairing, but it can at least limit the amount of poor choices that they can make. You can also force the CSS, but I realize that’s not ideal.

There’s also a feature in the block editor that warns users if there isn’t sufficient contrast in their pairings between text and background colors. While this isn’t the same as enforcing it, hopefully that can help.