Dimming objects behind marker when it is clicked/hovered upon?

Rather than layers, it might be easier to use a shape and its states. In the attached sample, I created a shape with two options in its New Shape, so you can see whether you prefer black or white to dim the background. Of course, you can change the color or transparency.

Just be sure the dimming shape (Rectangle 2) is above everything you want dimmed.  The shapes have superpowers, and that's certainly true in this case.