SGML says nothing about how a document should be displayed to the user, or rendered on paper. To do that, various languages have been developed to describe stylesheets, including DynaText, Panorama, SPICE, JSSS, FOSI, CSS, and DSSSL.
For DocBook, we are using stylesheets written in DSSSL. For XHTML we are using CSS.
The Documentation Project uses a slightly customized version of Norm Walsh's modular DocBook stylesheets.
These can be found in textproc/dsssl-docbook-modular.
The modified stylesheets are not in the ports system. Instead they are part of the Documentation Project source repository, and can be found in doc/share/xml/freebsd.dsl. It is well commented, and pending completion of this section you are encouraged to examine that file to see how some of the available options in the standard stylesheets have been configured in order to customize the output for the FreeBSD Documentation Project. That file also contains examples showing how to extend the elements that the stylesheet understands, which is how the FreeBSD specific elements have been formatted.