Changing Theme At RunTime

 private void Page_PreInit(object sender, EventArgs e)
    {
        if (Request.QueryString["thm"]== "T12")
            Page.Theme = "T12";
        else if (Request.QueryString["thm"] == "T14")
            Page.Theme = "T14";
        else
            Page.Theme = "T16";

    }

    protected void ddl_SelectedIndexChanged(object sender, EventArgs e)
    {

        if (ddl.SelectedIndex == 0)
        {
            Response.Redirect("Default.aspx?thm=T12");
        }

        if (ddl.SelectedIndex == 1)
        {
            Response.Redirect("Default.aspx?thm=T14");
        }

        if (ddl.SelectedIndex == 2)
        {
            Response.Redirect("Default.aspx?thm=T16");
        }


    }