W3Schools has an example of formatting XML with CSS at
http://www.w3schools.com/xml/xml_display.asp
It's pretty much as you'd expect, except that no display properties are implicit, as they are with HTML. eg with this:
<something>Here is some text</something>
You can go:
something {
display:block;
color:#000066;
background-color:#ededed;
}
and so on.
But the limitations are that CSS has little in the way of processing power, and what it does have is not well supported cross browser; you will very likely find that you want to do node manipulation, computation, sorting and such like; you may want to replace instances of one node with larger chunks of code - embeds, bits of javascript, lists and tables. CSS isn't up to the job, because that isn't what it was designed for.
What you want is XSL -
http://www.w3schools.com/xsl/default.asp