You might be able to do with something like this:

.LiveMessenger .modulecontent { text-align:center;}

but I'm not positive if it will work.

Another option is to set the width on the chat in the settings so that it takes up most of the available width, it will then look more centered.

Hope it helps,

Joe

It's is not already in there, just add it to your style.css file near the bottom.