How to use custom CSS to make nav items have a color background

Hi everyone!

I'm looking for some help formatting/customizing my nav bar for this new site. The website is not live yet so below a screen shot of the header currently.

What I'm looking to do is keep everything the same BUT I'd like to add a dark box around each nav item (Fat things, About, Modeling, Contact) to increase legibility - like a black block with yellow or white text - while keeping the pattern background.

Current header attached:

Hi,

What is site url? We can help easier