Need help styling nav menu

I have been trying without success to change the styling on my basic navigation menu. I just want really simple style changes, but when I look on Firebug to see where the relevant CSS is, I get all these different lines and layers of inherited code, and I don’t know which ones are the relevant ones. Someone else tried to help me, earlier today on here, and said that there is something blocking the CSS changes. I don’t understand. Should I take away my downloaded font, and empty out my child theme style sheet, and start over?

I have been studying the codex and forums, but with no success. I understand how to style simple lists from scratch with html, but can’t figure out how to penetrate WordPress’s php dynamic environment.

All I want is to be able to remove that black background to the nav bar, and just have words for the different page links, sitting on top of a horizontal line. And I want to style the colors and font size and so on.

Thanks for anyone’s help. My site is here.