// get the cute editor instance
    var editor1 = document.getElementById('<%=Editor1.ClientID%>');
    
    //Get the editor content 
    var editdoc=editor1.GetDocument(); 

    var head = editdoc.getElementsByTagName("head")[0];

    
    alert("Before:"+head.innerHTML);

 

    var style = editdoc.createElement(SomeStyleSheetPath);
    head.appendChild(style);

    alert("After:"+head.innerHTML);