It's from the category seperation. Go into your forumhome template and find this:
<!-- main -->
<table cellpadding="{tableouterborderwidth}" cellspacing="0" border="0" bgcolor="{tablebordercolor}" {tableouterextra} width="{contenttablewidth}" align="center">
and replace it with this code:
<!-- main -->
<table cellpadding="{tableouterborderwidth}" cellspacing="0" border="0" bgcolor="{pagebgcolor}" {tableouterextra} width="{contenttablewidth}" align="center">
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