How do I add a border color under my header with CSS?

Hi Andy,

You can try adding this CSS code.

.inside-navigation{border-bottom: 2px solid black;}

Here's a demo video on how it works.
https://share.getcloudapp.com/eDuB0LXN

If you want a full-width border, you can try.

#site-navigation {border-bottom: 2px solid black;}

Let us know if it works for you.

Fantastic! Thanks Elvin!

Nice one, No problem.:)