float in the center, regardless the page width

In Q&A plugin, front-end smaller window it shows the Q&A layout properly but in a wider page it's stays on the left-side. Please see screenshot here: http://f.cl.ly/items/2z053u2A463i2247452N/QA%20-%20screenshot.jpg

How do I make it float in the center, so regardless the width it adjusts automatically? Thanks in advance!