Change theme dynamically to master pages in asp.net

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ASP.NET

Hi

I have to apply theme for all pages i created to themes one is red and another one is orange its showing only default red if i changed in dropdownlist it's not changing i implemented code in master page
i wrote

protected void Page_PreInit(object sender, EventArgs e)
   {
       string thm;
       thm = (string)Session["themeSelected"];
       if (thm != null)
       {
           Page.Theme = thm;
           DropDownList1.Text = thm;
       }
       else
       {
           Session["themeSelected"] = DropDownList1.Text;
           Page.Theme = "Blue";
       }

   }

   protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
   {
       Session["themeSelected"] = DropDownList1.Text;
       Server.Transfer(Request.FilePath);
   }

and after i changed in web.confing <pages theme ="Orange">
where i did mistake?