.SIS_Mask_C
{
    position: fixed;
    width: 100%;
    height: 100%;
    top: 0;
    left: 0;
    background-color: rgba(133, 133, 133, 0.31);
     z-index: 90000000;
}
.SIS_Loader_C
{
    background: url('/Styles/Images/loader.gif') no-repeat;
    position: fixed;
    width: 400px;
    height: 400px;
    z-index: 100000000;
    top: 65%;
    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*/
        
}