Perhaps consider placing your primary content within a div and centering it using the following CSS:
margin: 0 auto;
width: /* specify the width of your content */
You may need to reorganize your HTML structure; when I attempted this, it resulted in cutting off your background. To resolve this, remove the background-image from the centered div and encompass it around that specific div. To clarify, here is an example:
<body>
<div id="bg"><!-- assign background-image to this ID in CSS -->
<div id="maincontent">
<!-- this div holds all your content that requires centering -->
</div>
</div
</body>
Next, in your CSS file:
#bg { background-image: /* blabla */ }
#maincontent { margin: 0 auto; width: /* specify the width as mentioned above */ }
Consider implementing something along those lines.