I highly recommend using FireFox and FireBug to understand and edit/preview your css. Once you are satisfied with the results in firebug, you will have to edit the category.css yourself in some editor. Firebug unfortunately doesnt support saving your changes.
That looks to be more for the Sub Category page. I was hoping to modify the menu on the side to have a different background color/image for the Sub Categories. This is to distinguish a Sub Category from a main Category selected.
This you can do by setting a new style class for the sub category links as they are set as the category links e.g(li_selected,li_unselected) You will need to modify the code behind to change this property as the menu is rendered dynamically.
Mike is right but there is a small problem. NOPCommerce CategoryNavigation control does not assign seperate css classes for root categories and sub categories.
however this can be achieved as follow
edit like 144 and line 146 of CategoryNavigation.ascx.cs
as zeeshanejaz posted in his code you will need to edit the code behind and add a new style for sub categorys and this will allow you to restyle the sub categorys.
I tried the code alright and rebuilt my site but I couldn't get it to work. Not 100% sure about how to add the style for the sub categories to the stylesheet. If you had a short example it would be great and then I could play with the style.