That's their CSS, hope that helps!

.col[data-type="project"] .ph span {
    background-color: white;
    top: 5%;
    left: 5%;
    right: 5%;
    bottom: 5%;
    width: 90%;
    height: 90%;
    -webkit-opacity: 0;
    -moz-opacity: 0;
    opacity: 0;
    -ms-filter: progid:DXImageTransform.Microsoft.Alpha(opacity=0);
    filter: alpha(opacity=0);
    -webkit-transition: all 400ms ease-out;
    -moz-transition: all 400ms ease-out;
    -ms-transition: all 400ms ease-out;
    -o-transition: all 400ms ease-out;
    transition: all 400ms ease-out;
}
  .col[data-type="project"] .ph span:hover {
    -webkit-opacity: 1;
    -moz-opacity: 1;
    opacity: 1;
    -ms-filter: progid:DXImageTransform.Microsoft.Alpha(opacity=100);
    filter: alpha(opacity=100);
  }