THIS GOES IN YOUR HEAD TAGS

<link id="pagestyle" rel="stylesheet" href="css/style.css" type="text/css" />

 <script>
 function swapStyleSheet(sheet){
  document.getElementById('pagestyle').setAttribute('href', sheet);
}
</script>

THIS GOES IN YOUR BODY SOMEWHERE


<button onclick="swapStyleSheet('css/style1.css')">Green Style Sheet</button>
    <button onclick="swapStyleSheet('css/style2.css')">Blue Style Sheet</button>
    <button onclick="swapStyleSheet('css/style.css')">Default Style Sheet</button>


REMEMBER TO EDIT THE CORRECT LOCATION OF THE STYLE SHEETS SUCH AS href="css/style.css"  AND ('css/style.css')