Navigation drop down menu style and get id or class

I got a question, how would you make a drop-down menu item, bold or hide. I am trying this one but it doesn't work (I am using 7.1)

a[href="/mega-menu"]+.header-nav-folder-content .header-nav-folder-item.header-nav-folder-item--external a:nth-child(5){
font-weight: bold;
}

  also this one is not working, nothing happens;

a[href="/mega-menu"]+.header-nav-folder-content a:nth-child(5){
font-weight: bold;
}

I would like the "Antioxidants" to bold and hide "Herbs"

Thank you!

PS. the attached image is my menu with two columns