XML documents can be styled using both CSS and XML stylesheets.
CSS stylesheets will not change the basic structure of the document but can give the XML elements the font, color, etc. presentation desired. This is described in a set of W3Schools lessons at http://www.w3schools.com/xml/xml_display.asp .
XSLT stylesheets will allow you to re-structure, filter, format, etc. the information. This is described at http://www.w3schools.com/xsl/xsl_transformation.asp.
Both of these can be used together. So for example, a common usage pattern is to start with an XML document, transform that document with one or more XSLT stylesheets, finally output an HTML document, and the XHTML/HTML document refers to a CSS stylesheet for the final presentation information (fonts, color, placement in the browser window, etc.)
I maintain several del.icio.us lists on this topic. See http://del.icio.us/brianlawler/references+CSS, http://del.icio.us/brianlawler/references+XSLT and http://del.icio.us/brianlawler/references+XML .