Hi there,
Please add custom class name like transparent-bg to sections that you want to add overlay, please add it in sections class field.
Add following code in Customize -> Custom -> CSS :
.transparent-bg{
position:relative;
}
.transparent-bg:before {
content: "";
position: absolute;
top: 0;
right: 0;
left: 0;
bottom: 0;
width: 100%;
height: 100%;
display: block;
background-color: rgba(0,0,0,0.5);
}
Hope it helps.