You can do this in public_common.pl or in the Control Panel...The latter is the easier option. In your CP go to Primary Settings/Style Templates/Forum Home Style Templates and as Burak pointed out in his example set the first Table set into your "Header" and the second Table set into your "Footer" slots.
In each "
" tag you can set the width in pixels or percentages. Remember to set your layout to or do so in the Table (e.g., align="center") code, because if you have a fixed width (e.g., 800) for your site it will position left by default. 
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