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):
<!-- Display center blocks if any -->
<{if $xoops_showcblock == 1}>
<table cellspacing="0">
<tr>
<td id="centerCcolumn" colspan="2">
<!-- Start center-center blocks loop -->
<{foreach item=block from=$xoops_ccblocks}>
<{include file="default/theme_blockcenter_c.html"}>
<{/foreach}>
<!-- End center-center blocks loop -->
</td>
</tr>
<tr>
<td id="centerLcolumn">
<!-- Start center-left blocks loop -->
<{foreach item=block from=$xoops_clblocks}>
<{include file="default/theme_blockcenter_l.html"}>
<{/foreach}>
<!-- End center-left blocks loop -->
</td><td id="centerRcolumn">
<!-- Start center-right blocks loop -->
<{foreach item=block from=$xoops_crblocks}>
<{include file="default/theme_blockcenter_r.html"}>
<{/foreach}>
<!-- End center-right blocks loop -->
</td>
</tr>
</table>
<{/if}>
<!-- End display center blocks -->
.
Specifics.
Where? Code for which blocks? Left Right or even the side menu bars? Which one and where?