bootbox 水平垂直居中

bootbox.alert('Danger!!' ).find('.modal-content').css({
    'background-color': '#f99',
    'font-weight' : 'bold',
    'color': '#F00',
    'font-size': '2em',
    'margin-top': function (){
        var w = $( window ).height();
        var b = $(".modal-dialog").height();
        // should not be (w-h)/2
        var h = (w-b)/2;
        return h+"px";
    }
});