Deliverance does transformations of HTML to 'theme' pages, similar in function to XSLT but using a simpler XML-based language to express the transformation.

http://www.openplans.org/projects/deliverance/introduction
Deliverance Introduction - deliverance - OpenPlans
http://www.openplans.org/projects/deliverance/specification
Specification for Deliverance - deliverance - OpenPlans