Please use the following in the Searchlight Options > Color > Color CSS of your Site Box . Change the rgba(15, 135, 75, .75) with your RGBA (where A is for transparency) Color Values. This Tutorial may help you for further assistance.
.triangle { background:#149755; width: 70%; height: 100%; position: absolute; z-index: 3;
background: -webkit-linear-gradient(-60deg, transparent 50%, rgba(15, 135, 75, .75) 50%);
background: -moz-inear-gradient(-60deg, transparent 50%, rgba(15, 135, 75, .75) 50%);
background: -o-linear-gradient(-60deg, transparent 50%, rgba(15, 135, 75, .75) 50%);
background: -ms-linear-gradient(-60deg, transparent 50%, rgba(15, 135, 75, .75) 50%);
background: linear-gradient(-60deg, transparent 50%, rgba(15, 135, 75, .75) 50%);
background-size: 110% 100%; }