Adding a gradient border at the bottom of the navigation menu

Hello,

I want to add a gradient border underneath my main navigation menu to create a visible separation from the content. Here's a great example of what I want to do: https://www.leafly.com/news/cannabis-101/what-is-thcv-and-what-are-the-benefits-of-this-cannabinoid

You can see there's a dark grey-to-light gray gradient border under the navigation. I imagine I need to add some kind of gradient/background CSS to my header or nav menu div.

Would appreciate any help