In theory this should already be possible, although I didn’t try it myself. Furthermore you would need to create the dimming functionality on every page in your project.
I use the dimming functionality for highlighting a specific part of the page and dimming other parts after clicking on a button, but I guess it can also be used with the Idle Event functionality (see Idle Events in 3.10.0).
The dimming is created by controlling the visibility (IsVisible property) of a transparent rectangle that covers the entire page and is on top of every other element. You have to experiment with the color and opacity of that rectangle.