bold the main category, but unbold the subcategory?

[diablo]

you have catcolors.css file
all tricks with categories you can do over there
if you know your category id you can even use some magic of php+ajax+html+cookies to make those categories expand and collapse
look into categories.tpl

[acidrain]

You can use this form to call your MAIN CATEGORY

<b>Category Main</b>

This is not a pretty rule, but it works perfect. The "problem" is that, category url is called "http://url/........=<b>Category Main</b>"

I think you can remove the tags with your .htaccess