CSS rounded corners for post windows

I added some changes to my style.css to round the left side window corners. The xhtml I have to implement it is:

<div class=”roundedcornr_box”>
<div class=”roundedcornr_top”><div></div></div>
<div class=”roundedcornr_content”>
/*Post is placed here*/
</div>
<div class=”roundedcornr_bottom”><div></div></div>
</div>

Now, what I really want is a way to add this to my index.php so that I don’t have to add the xhtml to all my posts. Is there a way to do so, because I haven’t had any luck so far. Thanks in advance.

Here is the section of my index.php that seemed like the addition would belong in, but I haven’t had any luck making a change that produces the desired result, and my knowledge of php is barely good enough to stumble around my theme with:

<?php while (have_posts()) : the_post(); ?>
<!– item –>
<div class=”item entry” id=”post-<?php the_ID(); ?>”>

<div class=”chronodata”><?php the_time(‘j F Y’) ?> <?php _e(‘at’); ?> <?php the_time() ?><!– by <?php the_author() ?> –></div>

<div class=”itemhead”>
<h3>” rel=”bookmark”><?php the_title(); ?></h3>
</div>
<div class=”storycontent”>
<?php the_content(‘Read more »’); ?>
</div>
<small class=”metadata”>
<span class=”category”>Categories: <?php the_category(‘, ‘) ?> </span> | <?php edit_post_link(‘Edit’, ”, ‘ | ‘); ?> <?php comments_popup_link(‘Comment (0)’, ‘ Comment (1)’, ‘Comments (%)’); ?>
</small>
</div>
<!– end item –>