This style is applyed to whole div with class comment-box and it contains not only comment-body but title and other elements. But if you want you can surround all block with some new div and move these styles some lines higher...
I mean that you can move code:
$comment_number = $this->getVar('comment-number', 1);
$thisurl = $this->getVar('thisurl', '');
$commentBoxIndentStyle = ($this->getVar('avatar') == 1) ? ' avatar-indent' : '';
if (isset($comment->usertype)) {
$commentBoxIndentStyle .= ' usertype-' . $comment->usertype;
}
$commentVote = intval($comment->isgood) - intval($comment->ispoor);
if ($commentVote != 0) {
$commentBoxIndentStyle .= (($commentVote >= 0) ? ' positive-' : ' negative-') . ($commentVote - ($commentVote % 5));
}
right after line:
// return all comment item
Change code:
?>
<div class="rbox"><div class="rbox_tr"><div class="rbox_tl"><div class="rbox_t"> </div></div></div><div class="rbox_m">
<?php
with (we've added <div> with style)
?>
<div class="<?php echo trim($commentBoxIndentStyle); ?>">
<div class="rbox"><div class="rbox_tr"><div class="rbox_tl"><div class="rbox_t"> </div></div></div><div class="rbox_m">
<?php
And change:
?>
</div><div class="rbox_br"><div class="rbox_bl"><div class="rbox_b"> </div></div></div></div>
<?php
with (extra </div> added)
?>
</div><div class="rbox_br"><div class="rbox_bl"><div class="rbox_b"> </div></div></div></div></div>
<?php
I did not tested this code but I hope it will work

I want to show you the idea...