Finally, you can overwrite existing templates or add new ones in the so called driver files:
These files are almost empty, actually they only import the really big ones tblatex.xsl, tbhtml.xsl and tbdocbk.xsl, which are files that you shouldn't modify.
In DocBook, such files are called “customisation layers”.
Modifications of these files are especially useful for own letter designs, because the default is for a certain Bugs Bunny. For that, redefine the "letter" template and – for HTML output – the "closing" template.