The StyleSheetTheme property cannot be set, please override the property instead.

protected void Page_PreInit(object sender, EventArgs e)
       {


               if (Session["Theme"] != null)
               {
                   //switch (Request.QueryString["Theme"])
                   //{
                   //    case "Green":
                   //        Page.Theme = "Green";
                   //        Page.StyleSheetTheme = "Green";
                   //        break;
                   //    case "Default":
                   //        Page.Theme = "Default";
                   //        Page.StyleSheetTheme = "Default";
                   //        break;
                   //}
                   string ThemeName = string.Empty;
                   ThemeName = (string)Session["Theme"];
                   Page.Theme = ThemeName;
               Page.StyleSheetTheme = "Green";



               }




       }

       public override string StyleSheetTheme
       {
           get
           {
               return base.StyleSheetTheme;
           }
           set
           {
               base.StyleSheetTheme = value;
           }
       }