How does the menu style override work

I really do not get how this over-ride works on the menu of the template itself.

I have asked it to put in a gk_menu on a itemID and component, but still it will not drop down

dev.open-parliament.co.uk

Can someone explain.

I have created a new div tag, which makes the menu horz, but thats it.
Thanks

Please read this article >> https://www.gavick.com/documentation/jo ... work-menu/