Hello,
This is really custom coding request but I quick wrote up the code for you. Just add to your custom.css file:
#s5_menu_inner {
float: left !important;
left: 50% !important;
margin: auto !important;
position: relative !important;
width: auto !important;}
#s5_nav li {
float: left;
position: relative !important;
right: 50% !important;}