themes implemantation dynamically

Hiii EveryBody.
first of all i would i like to thank all of you specially kirtan for kind support,
now again i struck in problem.
i have 2 skin file(Skinfile or SkinFile2), nd i set that code for dynamically change by user,
here is the code. on aspx page i linked my Skinfile (default)
and create 2 links
IstTheme

IIndTheme
and on aspx.cs page i called it,
protected void Page_PreInit(object sender, EventArgs e)
    {
       protected void Page_PreInit(object sender, EventArgs e)
    {
        if (Request.QueryString["id"] != null)
        {
            switch (Request.QueryString["id"].ToString())
            {
                case "First": Page.Theme = "SkinFile";
                    break;
                default: Page.Theme = "SkinFile2";
                    break;
            }            
        }
    }
        {
            switch (Request.QueryString["id"].ToString())
            {
                case "First": Page.Theme = "SkinFile";
                    break;
                default: Page.Theme = "SkinFile2";
                    break;
            }            
        }
    }
but nothing happen, id cannot passed. if (Request.QueryString["id"] != null) its always show false. please tell me where  i m making mistake
Thanks you
Regards
Rashid