There’s a theme component that makes adding links to the top nav really easy:
For custom styling, you can use that theme component and then target the link using the title attribute (which you configure in the theme component settings as the link description).
If you set up your new link’s description to be “This is an example title” you could target it using CSS like this:
.nav-pills>li[title="This is an example title"] {
background: blue;
>a { color: white; }
}