Nav Border

I have a navigation bar on my homepage and I want to make it longer, but when I increase the size from 960px to 1100 it doesn’t keep nav bar centered….it just adds the extra 140 pixels to the right side. Is there a way I can have it expand equally to both sides? Also, can you tell me how to round the edges of the nav bar. Currently it is sharp, block edges, I’d like to try them out with rounded. Thanks so much!