XML is a mark-up language document while CSS is a stylesheet. Thus you cannot convert a mark-up language document to a style sheet.
Probably what you want to do is use CSS in XML. To do this refer to :
"Building Documents with XML, XSL, and CSS"
http://www.siteexperts.com/tips/xml/ts01/page1.asp
XML.com: What Place Has CSS in the XML World?
or this http://www.xml.com/pub/2000/03/08/style/index.html
How to add style to XML (http://www.w3.org/Style/styling-XML)
or get
XML Editor and XSLT Debugger from
http://www.oxygenxml.com/
is a complete cross platform XML editor providing the tools for XML authoring, XML conversion, XML Schema, DTD, Relax NG and Schematron development, XPath, XSLT, XQuery debugging, SOAP and WSDL testing.