Need help finding the CSS for the header nave

I need help figuring out how to change the color via CSS on the nav on my theme, which is the Florida theme: http://themeforest.net/item/florida-premium-multipurpose-responsive-theme/6609201

I'm wondering if I can do it via my custom css file, or if I have to do it in their main.menu.css.

I have this part: header#header nav {background: #274b49
}

It' the menu items that I can't figure out.

Thank you,
Marla