I am one of many developers for PhpGedView http://phpgedview.sourceforge.net an open source genealogy program for the web.
In the past half year or so we moved to xhtml (transitional) and heavy use of CSS. Can any of you gurus out there check out any of the sites listed in http://phpgedview.sourceforge.net/registry.php as being 3.1 or newer and offer some advice? Some of the pages have some very complex layouts such as the pedigree charts.
From most sites you can change the theme (though nothing like the Zen Garden). Theme changes involve much more than just changing CSS. Something to remember is that changes often involve changing multiple files. One area that we developed was full RTL support (only Hebrew translation for now) using mostly CSS (though it sometimes involves dynamic code). This was a non trivial undertaking.
Since it is open source feel free to browse the source code.
Comments can also be posted at https://sourceforge.net/forum/forum.php?forum_id=185165