you need to edit your theme.html and place this
<div><{$xoops_contents}>div>
above the code for displaying blocks (example code from default theme):
<{if $xoops_showcblock == 1}>
<table cellspacing="0">
<tr>
<td id="centerCcolumn" colspan="2">
<{foreach item=block from=$xoops_ccblocks}>
<{include file="default/theme_blockcenter_c.html"}>
<{/foreach}>
td>
tr>
<tr>
<td id="centerLcolumn">
<{foreach item=block from=$xoops_clblocks}>
<{include file="default/theme_blockcenter_l.html"}>
<{/foreach}>
td><td id="centerRcolumn">
<{foreach item=block from=$xoops_crblocks}>
<{include file="default/theme_blockcenter_r.html"}>
<{/foreach}>
td>
tr>
table>
<{/if}>
.
Specifics.
Where? Code for which blocks? Left Right or even the side menu bars? Which one and where?