Windows are slightly "transparent" in such a way I can see underlying windows show through slightly to the current window
That's one of the "features" of the Mac O/S that I always thought, "what were they thinking?" Unfortunately I don't have an answer to that but to your question:

Go to System Preferences -> Accessibility -> Display

look for two check boxes: Increase Contrast and Reduce Transparency.

Adjust either or both to get your desired effect, that is not seeing the stuff behind a window.