It is possible to style XML with CSS and with XSLT. Most browsers will apply both CSS and XSLT stylesheets and render the results, when the proper processing instruction is provided in the XML.
To apply CSS to an XML file add a processing instruction like this:
<?xml-stylesheet type="text/css" href="my-style.css" ?>
To apply XSLT to an XML file add a processing instruction like this:
<?xml-stylesheet type="text/xsl" href="my-transform.xsl" ?>
If your styling requirements are more display and decoration of the XML content, rather than transformation, then CSS styling might be easier.
Check out this W3C page: How to add style to XML