Container not centered

[Fleiding]

I'm using the following CSS in additional.css to create a container around my forum (div opening in header and closing in footer):

#main_container {
    width: 1200px;
	max-width: 100%;
	margin: 0 auto;
	overflow: hidden;
	background: #FFFFFF;
    padding: 10px;
	border-left-style:solid;
	border-left-width:1px;
	border-left-color:#f4f4f4;
	border-right-style:solid;
	border-right-width:1px;
	border-right-color:#f4f4f4;
}

As you can see in this picture (or forum) the container is slightly centered off to the right. I'm using the same CSS without any issues on WordPress. Can anyone help me with centering the container?