I did it with a conditional statement based on the category's "Image" name.

Each one of my root categories has it's own (unique) icon:

<%if Image eq'shop.jpg'%>

<a class="category_links" href="<%URL%>">

<%Full_Name%>">
</a>


<%elseif Image eq'forum.gif'%>
<a class="category_links" href="<%URL%>">
<%Full_Name%>">
</a>

... etc


<%else%>
<a class="category_links" href="<%URL%>">
<%Short_Name%>
<%if Related%>
@
<%endif%>
</a></b> <i><small>^{(
<%Number_of_Links%>
)}</small></i>
<%endif%>


I left out the <%Number_of_Links%> tags for the root categories.

If you do not have unique icons (my 'Image' field) for the top categories, you could do this:

<%if Category_Template eq home.html%>

<a class="category_links" href="<%URL%>">

<%Full_Name%>">
</a>

<%else%>
<a class="category_links" href="<%URL%>">
<%Short_Name%>
<%if Related%>
@
<%endif%>
</a></b> <i><small>^{(
<%Number_of_Links%>
)}</small></i>
<%endif%>



Let me know if this makes sense.