Hi Prince,
to add new css file..
right click on your project - Add new item - select stylesheet - click Ok
to add existing .css file..
write like this
.body
{
background: #b6b7bc;
font-size: .80em;
font-family: "Helvetica Neue" , "Lucida Grande" , "Segoe UI" , Arial, Helvetica, Verdana, sans-serif;
margin: 0px;
padding: 0px;
color: #696969;
}
and in aspx page you can refere
<link href="CSS/StyleSheet.css" rel="stylesheet" type="text/css" />
Tank U.