Cenetering board problem please help

[Zhenson]

Alrighty so I have a site actually two that I have the same codes on, so the problem is, I cenetered the homepage, and boards, but when you go to post a topic or reply to something it stretches the board and looks unappealing, now I don't know what the problem is and was hoping someone could help us or else we will have to go with redoing the hole thig, now heres the link:

lettherainfalldown.proboards.com/index.cgi