Center Menu Bar

Yes, you can use CSS in a child theme or a CSS plugin (like Jetpack) to do this. I recommend using Firebug for CSS work–makes it much easier.

I think this should do it, but it’s untested — you may need to play around with it a bit.

header#masthead .menu ul li,
header#masthead ul.menu li {
   display: inline-block;
   float: none;
}
.main-navigation {
   text-align: center;
}