Hi Glenn,
My advice is view the source of the rendered page and look at the markup and consider whether there might already be some CSS rules being applied.
In this case the ul has a class "linkitem" (probably not the best choice of class name there), and the li elements alternate with class "linkitem" and "linkaltitem".
So if there are CSS rules somewhere in your CSS for those then you need to override them. You custom CSS should go lower in the file (ie list your custom file at the bottom of style.config to make it come in lower) and the selectors should be a little more specific like:
.storecats ul.linkitem { }
.storecats ul.linkitem li.linkitem, .storecats ul.linkitem li.linkaltitem { }
in order to be sure it overrides any CSS rules above it.
Hope that helps,
Joe