How to REMOVE horizontal line on header nav menu

Hello. I am no coder at all but here I am creating a website! I'm on Wordpress WP Dating Child Theme and there's this horizontal line that appears above the header menu.

So it pretty much looks like this:
_____________________________________________
HOME CONTACT US PHOTO GALLERY

When I add more menu items, the line gets longer and then creates a second row. I've changed the border to NONE and it still hasn't removed the line. This is the 12th forum I've joined the previous forums gave me some codes to put in ADDITIONAL CSS but none of them worked.

Can someone help me out?

Thanks so much.