Changing the Color of the Pop Up Overlay Window Background

Please paste this code below to Page Settings >> Other Scripts >> choose Custom CSS from the dropdown


.op-popup-fancybox .fancybox-inner, .op-popup-fancybox .fancybox-outer {
background-color: #000;
}
.fancybox-opened .fancybox-skin {
background-color: #000 !important;
}

You can change the value of "#000" to your desired background color.