Looking at your site with
Firebug, it appears that the "bottom center" positioning is failing to be applied. Everything should function exactly the same between both versions of the framework, are you sure that you don't have another option set differently?
You may also want to try applying the
Vertex Upgrade Patch, just in case something was updated recently that resolves the issue.
If that doesn't solve the problem, you can always add the following code to the end of the custom.css or style.css file for the theme:
body {
background-position: bottom center !important;
}