Adapting chapter titles for HTML output

[Boris <boriss@web.de>]

I'm a DocBook newbie playing around with the publishing process currently.  
I'm using DocBook 5 and xmlto on Cygwin which seems to work fine to create  
HTML documents. However I need to customize the output a bit more.

For example a chapter title is converted to "Chapter 1. Bla" by default.  
However I really need something like "<span class='chapter'>Chapter  
1:</span> Bla" (please also note the colon). I went through the DocBook  
XSL Stylesheets User Reference but couldn't find any parameter which I  
could set to adapt chapter titles. I guess I need to tweak the XSL  
stylesheets manually? However I've no idea where to start looking. Is  
there any documentation how to get into the stylesheets and find at least  
the file which probably contains the XSL to adapt?

Boris