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

Can you share link to your site? We can check easier.