Using one of the built in Math functions of PHP we can easily create zebra striped lists.
Let's say we have an array of entries stored within the $comments variable.
All we have to do with this is to use the fmod function and assign a css class that alternates per entry.

<ul>
<?php foreach ($comments as $int => $comment): ?>
<li class="<?php echo fmod($int, 2) ? 'even' : 'odd' ?>">
<?php endforeach; ?>
</ul>

Then just give the even and odd classes different colours to suit your layout.

li.even {
background-color:#FFFFFF;
color:#000000;
}

li.odd {
background-color:#000000;
color:#FFFFFF;
}


And you're done!