Why not try a floating group that is stretched out over the entire page and the background color set to how you want it. You have two options from here:
Keep your menu inside another floating group, just make sure that you move it to the front so that it floats on top of the background above. Just add a conditional to the background floating group that it is visible when the side menu is visible.
You place the menu inside a regular group and put it on the side of the floating group you added, just change the background color of the group to the color of the side menu and then show the background floating group to show it.