if (screen.width <= 800)
  {
     document.write("<link rel='stylesheet' type='text/css' href='main_menu/web_intern1.css' >");
   }
else
   {
     if (screen.width <= 1024)
       {
         document.write("<link rel='stylesheet' type='text/css' href='main_menu/web_intern2.css' >");
       }
     else
       {
         document.write("<link rel='stylesheet' type='text/css' href='main_menu/web_intern3.css' >");
        }
    }