Alternating comment background colour

I’m trying to fix alternating comment background colours on my blog using the Aeros theme but so far had no success. I just can’t figure out where to put what in the code and most of the guides have different instructions. Is anyone able to figure out how to get it to work?
Here is the comments.php code:

<?php if ( !empty($post->post_password) && $_COOKIE['wp-postpass_' . COOKIEHASH] != $post->post_password) : ?>

                 <p><?php _e('Enter your password to view comments.'); ?></p>

         <?php return; endif; ?>

	<h2 id="comments"><?php comments_number(__('No Comments'), __('1 Comment'), __('% Comments')); ?>
	<?php if ( comments_open() ) : ?>
	         <a href="#postcomment" title="<?php _e("Leave a comment"); ?>">&raquo;</a>
	<?php endif; ?>
	</h2>

<script language="javascript">

function setCommentFormValue(creator){
	if(document.forms["postloginComment"])
		document.forms["postloginComment"]["body"].value = creator;
	else if(document.forms["postComment"])
		document.forms["postComment"]["body"].value = creator;
}

</script>

<?php if ( $comments ) : ?>

	<ul id="commentlist">

	<?php foreach ($comments as $comment) : ?>
	         <li>

	         <!--Change the gravatar size by replacing '70' with bigger resp. smaller sizes-->
                 <div class="authorcomm">

                 <?php echo get_avatar( $comment, 70 ); ?>

                 </div>
                 <div class="authorname"><?php comment_author_link() ?><div class="com_date"><?php comment_date('G:i') ?> <span><?php comment_date('j F') ?></span> <?php if(function_exists("yus_reply")) yus_reply(); ?><?php edit_comment_link(__("Redigera"), '  '); ?></div></div>
	         <?php comment_text() ?>
<div class="meta"></div>

	         </li>

	<?php endforeach; ?>

	</ul>

<?php else : ?>

<?php endif; ?>

<?php if ( comments_open() ) : ?>
	<h2 id="postcomment"><?php _e('Leave a comment'); ?></h2>

<?php if ( get_option('comment_registration') && !$user_ID ) : ?>
<p><?php printf(__('You must be <a href="%s">logged in</a> to post a comment.'), get_option('siteurl')."/wp-login.php?redirect_to=".urlencode(get_permalink()));?></p>
<?php else : ?>

	         <form action="<?php echo get_option('siteurl'); ?>/wp-comments-post.php" method="post" id="commentform" name="commentform">

	         <?php if ( $user_ID ) : ?>

	         <p><?php printf(__('Logged in as %s.'), '<a href="'.get_option('siteurl').'/wp-admin/profile.php">'.$user_identity.'</a>'); ?> <a href="<?php echo get_option('siteurl'); ?>/wp-login.php?action=logout" title="<?php _e('Log out of this account') ?>"><?php _e('Log out &raquo;'); ?></a></p>

	         <?php else : ?>

	         <p><input type="text" name="author" id="author" value="<?php echo $comment_author; ?>" size="22" tabindex="1" />
	         <label for="author"><small><?php _e('Name'); ?> <?php if ($req) _e('(required)'); ?></small></label></p>

	         <p><input type="text" name="email" id="email" value="<?php echo $comment_author_email; ?>" size="22" tabindex="2" />
	         <label for="email"><small><?php _e('Mail (will not be published)');?> <?php if ($req) _e('(required)'); ?></small></label></p>

	         <p><input type="text" name="url" id="url" value="<?php echo $comment_author_url; ?>" size="22" tabindex="3" />
	         <label for="url"><small><?php _e('Website'); ?></small></label></p>

	         <?php endif; ?>

<?php live_preview() ?>
	         <p><textarea name="comment" id="comment" cols="100%" rows="10" tabindex="4"></textarea></p>
<span style="float: right;">
	         <p><input name="submit" type="submit" id="submit" align="right" tabindex="5" value="<?php echo attribute_escape(__('Skicka')); ?>" />
	         <input type="hidden" name="comment_post_ID" value="<?php echo $id; ?>" />
	         </p></span>
	         <?php do_action('comment_form', $post->ID); ?>

	         </form>

<?php endif; // If registration required and not logged in ?>

<?php else : // Comments are closed ?>
<p><?php _e('Sorry, the comment form is closed at this time.'); ?></p>
<?php endif; ?>