(b)Subcategories(/b)(br /) (div class="dircategs") (table ) (tr) (td)(a class="dirlink" href="tiki-directory_browse.php?parent=1")CSS(/a) (br /) Cascading Styles Sheets -- Resources, information, tutorials, etc. (/td) (td) ..
So the categories are in a table in div.dircategs. You can give properties to div.dircategs and div.dircategs table and div.dircategs table td.
Sure, there's no need to include every selector in a stylesheet, if it isn't necessary to treat the item individually. I didn't specify a dirlink style in Tikipedia, for example, because there's nothing unique about those links; they follow the global link style. It reduces the CSS file size to only include things as necessary. Tiki's CSS files are huge, so should be trimmed whenever possible.
If the CSS file you're using doesn't specify anything for a particular selector in the page that you want to change, you can add that yourself.