Usually you should ask a question like this on the VBAdvanced forum since it's their product... But I recently did something like this myself.
Edit your spacer_open template and use this code instead:
<!-- open content container -->
<if condition="$show['old_explorer']">
<table cellpadding="0" cellspacing="0" border="0" width="$stylevar[outertablewidth]" align="center"><tr><td class="page" style="padding:0px $stylevar[spacersize]px 0px $stylevar[spacersize]px">
<else />
<div align="center">
<if condition="THIS_SCRIPT=='adv_index'">
<div class="page" style="width:1200px; text-align:$stylevar[left]">
<else />
<div class="page" style="width:$stylevar[outerdivwidth]; text-align:$stylevar[left]">
</if>
<div style="padding:0px $stylevar[spacersize]px 0px $stylevar[spacersize]px" align="$stylevar[left]">
</if>
The part in red is what I changed.