As you know, you can insert some content in any HTML page. You can insert content by using document.write() or by using the property innerHTML in a DOM object. Also, you can use an existing TAG in order to add/remove elements from its. For example, you can add a Javascript with:

function addScripting(s) 
{
 var scriptNode = document.createElement('script'); 
 document.getElementsByTagName("head")[0].appendChild(scriptNode); 
 scriptNode.language='javascript'; 
 scriptNode.src=s;
}

Of course, you can also add a CSS style with:

var cssNode = document.createElement('link'); 
cssNode.setAttribute('rel', 'stylesheet'); 
cssNode.setAttribute('type', 'text/css'); 
cssNode.setAttribute('href', cssfile ); 
document.getElementsByTagName('head')[0].appendChild(cssNode);