Hi,
If you check the HTML code of the page where the map is displayed (Firebug, chrome/Safari debugger), and find the section where the map is defined, you'll see a 'div' container with a 'map_canvas' ID. So, either you apply the css to this div so :
#map_canvas {margin: 0 auto;float:none;}
or to the parent div depending on how you've set the width of the map. If you set a with in % in the template configuration, you should apply the css to the parent div of the map.
I hope this helps.
Happy new year,
Olivier