I'll bet your browser is Netscape. Netscape does not yet have full support for CSS. In particular, tables (the tree generates a table) do not inherit many style properties. Try it with MSIE and you may see what you expected. I understand that Netscape's implementation of style inheritance will be fixed with version 5.0.
Hello Zopisti,
I hope you'll tolerate another question on navigation trees. Actually, it's related to the ZBook tutorial at ZDP. In the example, the tree is constructed within a <span class="tree"> tag. When displayed, the page renders with the tree in the prescribed style just as it should. However, when I implement the same tree syntax and the same style settings, my navigation tree displays "normal" fonts. I know my style settings are OK because I tested the <span class="tree"> tag elsewhere successfully.