How do I centre the logo and/or navigation?

Go to Theme Options > CSS Settings and add:

#header {
text-align: center;
}

#logo {
float: none;
}

#nav {
float: none;
position: static;
display: inline-block;
margin-top: 30px;
text-align: left;
}

#bp-buttons {
position: absolute;
top: 20px;
right: 20px;
width: auto !important;
}