.modal-backdrop
{
position: fixed;
top: 0;
right: 0;
bottom: 0;
left: 0;
z-index: 1040;
background-color: #000000;
opacity: 0.3;
filter: alpha(opacity=30);
}
.modal
{
position: fixed;
top: 50%;
/*left: 50%;*/
z-index: 1050;
overflow: auto;
/* this width is your loading img's width */
/*width: 100px;*/
/* please change the margin-top and margin-left here
* make sure the margin-top equals 1/2 loading img's height
* and the margin-left equals 1/2 loading img's width */
margin: -50px 0 0 -50px;
}