Altere o arquivo CSS com JS

<!DOCTYPE html>
<html>
  <head>
    <title>Changing CSS extern file using only JavaScript</title>
    <link id="changeCSS" rel="stylesheet" type="text/css" href="positive.css"/>
    <script type="text/javascript">
      function changeCSS() {

        var oldlink = document.getElementById("changeCSS");
        var cssFile;

        if(oldlink.getAttribute('href') === 'positive.css') {
          cssFile = 'negative.css';
        }
        else {
          cssFile = 'positive.css';
        }

        var newlink = document.createElement("link");
        newlink.setAttribute("id", "changeCSS");
        newlink.setAttribute("rel", "stylesheet");
        newlink.setAttribute("type", "text/css");
        newlink.setAttribute("href", cssFile);

        document.getElementsByTagName("head").item(0).replaceChild(newlink, oldlink);
      }
    </script>
  </head>
  <body>
    <button onclick="changeCSS();">change</button>
    <div class="box">Lorem ipsum dolor sit amet, consectetuer adipiscing elit.</div>
  </body>
</html>
Meti