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%;
}