Style sheets

This might be stupid question, but I cannot file immediate answer and
I hope most of the people on this ML have dealt with it.
With my old group of HTML files I had CSS file which I could include
as server-side include. Now, when we moved to XSL, which suppose to
replace/enhance CSS, I cannot find trivial way to define all my styles
within a single file and include into each XSL file. If I add <style>
element to XSL file, style is accepted, but if I try to include *.css
file, which contains already defined set of styles, I get an error from
XSL processor saying that document is not well formed.
What is the correct way to move from CSS files to XSL file? Should I rename
my css files into xsl file, add <style> element and use it as include into 
new xsl files? Or can I directly reuse CSS files as is?
Thanks a lot for any clue!

Shimon

 XSL-List info and archive:  http://www.mulberrytech.com/xsl/xsl-list