Example of a basic CSS media query.

media_queries.css

@media all and (max-device-width: 480px) {

  //
  // Define new CSS rules here for screens
  // that are less than 480px wide.
  //
  // These rules will need to OVERRIDE the global
  // styles that you have previously declared.
  //

  body {
    font-size: 12px;
    background-color: #fff;
  }

}