The ul is centered within the nav element with
margin: 0 auto
Additionally, I gave the ul a max-width of 800 px. The buttons are positioned to the left and right of the ul and the have set widths, so the space between them depends on the overall width of the ul.
If you would like the left and right buttons to be closer together, so that the space is smaller, you can simply give the ul a smaller max-width.