Styling one menu item differently

Edit the menu item, open the Link Type tab, and fill in Link CSS Style. Whatever you type there lands on the link as a class. That is the whole mechanism.

What you get

The class is applied to the anchor itself, so your stylesheet can do the rest:

.nav a.cta {
    background: var(--accent);
    color: #fff;
    border-radius: 100px;
    padding: .6rem 1.2rem;
}

Put cta in the field and that one item is now a button, everywhere the menu is rendered.

The rest of the tab

Two neighbours are worth knowing about. Link Title Attribute sets the tooltip, and Link Image puts an image in the item without touching a layout. For an icon next to one entry that is quicker than any override.

Where it stops being the right answer

This styles a link. It cannot change the markup around it, add a second line of text, or restructure the dropdown. The moment you need any of that you are looking at a mod_menu override, or at building the navigation in your template directly from the menu tree.

But for "make this one stand out", which is what people usually want, the field is already there and it takes ten seconds.

A note on naming

Use a class name that says what the item is, not what it looks like. cta survives a redesign; orange-button becomes a lie the first time the brand changes.