#1, wrap your gallery code in a div like this:
<div id="outerWrapper">
(gallery code here)
</div>
Then add this CSS to your page. The margin left/right set to auto centers it.
<style type="text/css" media="all">
#outerWrapper { width: 895px; margin: 0 auto; }
</style>
#2, open the galleries/ribbon_fullsize/css/dark.css file and make the changes in there. The top navigation links are styled in lines 18-28.
-justin