detect salesforce lightning experience - add stylesheet dynamically

gistfile1.txt

if( isLightningExperienceOrSalesforce1() ) {
      //add the lightning stylesheet to the header
     link = document.createElement( "link" );
     link.href = "{!URLFOR($Resource.SLDS0121, 'assets/styles/salesforce-lightning-design-system-vf.css')}"
     link.type = "text/css";
     link.rel = "stylesheet";
     link.media = "all";
     document.getElementsByTagName("head")[0].appendChild(link);
  }
  else {
      // Use classic Visualforce
  }

  function isLightningExperienceOrSalesforce1() {
    return((typeof sforce != 'undefined') && sforce && (!!sforce.one));
  }