change background to dark on load

I would like the background to be dark on load. i tried using the followng code but the color does not change when i use the drag function on the image.

.twentytwenty-overlay{
	background: none repeat scroll 0 0 rgba(0, 0, 0, 0.5) !important;
}

https://www.ads-software.com/plugins/twentytwenty/