How to Add a Background Color to NAV Menu

Hi,

I wanted to know how I can add a background color to my NAV Menu. My website is https://sites-see.dk

I would like to have something like this https://example.typepad.com/kb/images/2008/10/24/navbar_navlistitema.gif

If you can provide me the code also let me know where I enter it. can I use it in the custom CSS?

Thanks