body {
margin: 0;
background-color: #FFFFFF;
text-align: center;
}

#flash {
margin: 0;
padding: 0;
border: 0;
width: 800px;
height: 600px;
position: absolute;
left: 50%; 
margin-left: -400px;
top: 50%;
margin-top: -300px;
}