changing theme style and content

hey. i’m new in this world so forgive my nooby questions.
I’m using Parabola theme, and I want to edit the style of the navigation menu in the header. I want the buttons to fill the whole width of the header. that’s it… I tried to use firebug, and i kind of succeeded to do it when I see the page on my browser with the firebug, but i can’t find the actual code on the code editor and to replace it.