hi, you can do it like this,
//client side
<head id="Head1" runat="server">
<link rel="stylesheet" type="text/css" id="idStyle" runat="server" />
</head>
//server side
protected void Page_Load(object sender, EventArgs e)
{
if(yourcondition)
{
idStyle.Attributes.Add("href", "CSS/test1.css");
}
else
{
idStyle.Attributes.Add("href", "CSS/test2.css");
}
}
hope it helps.