Take a look at the source code on the showthread page. The part in red indents all the postbits by 15px on both sides:
Code:
<!-- open content container -->
<div align="center">
<div class="page" style="width:1000px; text-align:left">
<div style="padding:0px 15px 0px 15px">
<div id="edit1346621" style="padding:0px 0px 6px 0px">
<!-- this is not the last post shown on the page -->