Centering/positioning thumbnails

I would like to be able to alter the appearance of my thumbnails (for example, centering them beneath the slideshow). How can I do that?

The shortcode I’m using:
[jj-ngg-jquery-slider gallery="3" html_id="vehicle_slide" effect="fade" controlnavthumbs="nextgen_thumbs" directionnav="false" thumbsgap="10" manualadvance="true" order="sortorder" thumbscontainerheight="75" height="410" center="1"]