.SIS_Mask_C {
    position: fixed;
    width: 100%;
    height: 100%;
    top: 0;
    left: 0;
    background-color: rgba(133, 133, 133, 0.31);
}

.SIS_Loader_C {
    background: url('/images/loader.gif') no-repeat;
    position: fixed;
    width: 400px;
    height: 400px;
    z-index: 100000000;
    top: 55%;
    left: 55%;
    margin-top: -15em; /*set to a negative number 1/2 of your height*/
    margin-left: -15em; /*set to a negative number 1/2 of your width*/
}