Nest your popup in a parent element (div). Give this parent a z-index lower than the one of the popup. Make the parent expand all the way of the browser by setting its dimensions like 100VW and 100VH (type-in the vw and vh units, they mean "percentage of the viewport", webflow will take them). Center you popup inside the parent element.
Now, give the new parent element the onClick closing interaction you've been building for your "X" icon and it should close the popup as if you'd clicked on "X".
And now that you have this element you can try to give it a background color at 98% opacity, or a transparent texture from http://www.transparenttextures.com/, or both, to make you popup stands out