change menu font/size

My primary menu (nav bar?) sits under my header on a dark gray bar. I have tried everything but it will not let me change the font and size of the menu text. (right now it's white text, all caps, but tiny and not sure what font) I'd like menu items to show up in Oswald, all caps, white text, at a size 20. Is there a bit of CSS that might let me override the theme settings? Thanks so much for any help! Kady