if (screen.width <= 800)  {
  document.write('<link rel="stylesheet" type="text/css" href="basic_800.css">');
}
if (screen.width > 800 && screen.width <= 1024)  {
  document.write('<link rel="stylesheet" type="text/css" href="basic_1024.css">');
}
if (screen.width > 1024 && screen.width <= 1280)  {
  document.write('<link rel="stylesheet" type="text/css" href="basic_1280.css">');
}
if (screen.width > 1280)  {
  document.write('<link rel="stylesheet" type="text/css" href="basic.css">');
}