Adding line below fixed header

I’m trying to add a thin gray line right underneath my header, and I’m not sure what kind of coding would be the best to use for this. I’ve already tried @media all and (min-width: 50em) { .menu-primary-container { border-bottom: 1px solid #000; } but it didn’t work.

Additionally I’m trying to make the header fixed, but adding a “position: fixed” didn’t work either.

Any tips?

The theme I am currently using is Apex.