Change Colors in Menu (style.css?)?

I want to change the menu colors of my theme. The top part of the page is black background, and I want it white. (My logo is white background, you can see at http://www.teamworkcentral.com.)

It looks like I can go in and customize this with #primary-nav in style.css. Is that right? Is there an easier/better way I am missing?

Also, how do people generally handle CSS , should I just modify style.css and upload, should I maintain a separate file that inherits/overrides? Other?

My site is http://www.teamworkcentral.com if that’s helpful in answering.

-billb