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The problem definately stems from the theme.html file, as I can also see some layout problems in the main page, which are translated and highlighted in the forums. I hazard to guess that if you include a centre block in the front page, you will experience very similar problems.
Saying exactly what you've done wrong is a little difficult without viewing the theme.html source on it's own though, but it does look like the columns are handled badly which is causing the extra width. For example, on the forums it looks as though there is a left column defined somewhere with no content, creating the black space on the left hand side.
My first suggestion would be to have a look at all the <td> commands, and double check what they all do...