Well, the first thing you may consider is not center the document. Normally if you expand the window, the entire
document gets wider...it doesn't shift with a constrained size. You
must be fixing the width of the document rather than allowing it to
grow/shrink with the browser window. This is one of the features of HTML documents, the ability to resize and fit nicely based on the user's view.
That said, if you examine the rendered HTML you should see that the
buttons are placed in a <p class="buttons"> tag. So you could
use CSS to control them like all HTML, such as adding this to your
document (in source mode):
<style type="text/css">
p.buttons { text-align: center; }
</style>Update the programming guide for button message sets (the most likely place someone may look for this issue) to describe this, too:
http://code.google.com/p/openesignforms/wiki/ProgrammingGuide#Button_message_sets