adding a border color specific to nav bar link

hello there !

first of all here is how it looks like :

THE SITE

do you see the nav bar above page content ?
it is defined by div catnav.
elements inside of it are called cat-item-“n”

Originally i wanted to have the color of the top border of this nav in a specific color each time i would change category (links in menu, so cat-item-“n”). over the whole bar width.

There’s a class called ‘current-cat’, which lets you specify for instance the size of font for the selected ‘link’ you clicked on. Yet I couldn’t find a way to work it out so as to have a different top border color for catnav (class for nav bar), everytime i would click on a different cat-item-“n”

I would still very much like to do this.. and i’m stuck now :/

More generally i would like -as a principle- to separate my boxes on the page with such top border, and this every time I change category, with a top border of the specific color of my cat-item-“n”..
I’m really not sure how to achieve this..
I hope I’m being clear :
one link in nav clicked on = one color for top borders color everywhere on the page !

Any hint would be greatly appreciated.

thanks in advance !