There is an overlay option inside the fullwidth section module. This solution is for a section that doesn't have any overlay.

Or else, add this to the custom css section » before:

/* -- START add overlay color -- */
position: absolute;
top: 0;
left: 0;
width: 100%;
height: 100%;
content: "";
background-color: rgba(0,0,0,0.6);
/* -- END add overlay color -- */
Code language: JavaScript (javascript)