I would find the supersized.css file and edit #supersized for opacity
#supersized {
display: block;
position: fixed;
left: 0;
top: 0;
overflow: hidden;
z-index: -999;
height: 100%;
width: 100%;
opacity:0.6;
filter:alpha(opacity=60); /* For IE8 and earlier */
}
If you wanted to make specific opacity for specific slides you could make more specific selectors and alter those to your liking.