How to shorten the nav bar in Oxyccen template

Hi im running PunBB 1.2.17 Oxygen template

On my site the nav bar runs across the page 100% length
I want to shorten it to 990px across, in the center of the page. My logo is 990px wide, so this would line up with my logo

I cant figure out how to do this exactly.

I played around with site_forum.css and cant figure out how to shorten the length of the nav bar

so right now my nav bar is like this:
__________________________________________________________________________________________________

and I want to make it this:
                                            _________________________________________________________

thanks