Centering Navigation Bar in Custom Theme

Can anyone help me with centering the navigation bar for my site? I have contacted the theme designer but he has not responded. I currently have my main navigation menu (About, Blog, Galleries, Pricing, FAQ, Contact) and my categories menu (Weddings, Engagements, Portrait, Travel). I am getting rid of the second menu and instead want to include “Categories” as a dropdown in the main menu and then want to center the entire thing. Any help would be appreciated!

Website: http://www.jessicabosse.com