Tu peux pomper le CSS de mon site.
div.modal {
position: fixed;
top: 50%;
left: 50%;
z-index: 1050;
overflow: auto;
width: 560px;
margin: -250px 0 0 -280px;
background-color: #fff;
border: 1px solid #999;
border: 1px solid rgba(0,0,0,0.3);
-webkit-border-radius: 6px;
-moz-border-radius: 6px;
border-radius: 6px;
-webkit-box-shadow: 0 3px 7px rgba(0,0,0,0.3);
-moz-box-shadow: 0 3px 7px rgba(0,0,0,0.3);
box-shadow: 0 3px 7px rgba(0,0,0,0.3);
-webkit-background-clip: padding-box;
-moz-background-clip: padding-box;
background-clip: padding-box;
}
div.modal.fade {
-webkit-transition: opacity .3s linear, top .3s ease-out;
-moz-transition: opacity .3s linear, top .3s ease-out;
-o-transition: opacity .3s linear, top .3s ease-out;
transition: opacity .3s linear, top .3s ease-out;
top: -25%;
}