It’s counter intuitive but, there is no such thing as comments list template, ie the template file where you can define each comment look and feel. Of course (like with almost anything in WordPress) it’s possible to change comments list HTML, but it requires a bit different approach.

First in your theme functions.php file you need to define new function (you can call it anyway you want):

<?php
function mytheme_comment($comment, $args, $depth) {
   $GLOBALS['comment'] = $comment; ?>
   <li <?php comment_class(); ?> id="li-comment-<?php comment_ID() ?>">
     <div id="comment-<?php comment_ID(); ?>">
      <div class="comment-author vcard">
         <?php echo get_avatar($comment,$size='48',$default='<path_to_url>' ); ?>
 
         <?php printf(__('<cite class="fn">%s</cite> <span class="says">says:</span>'), get_comment_author_link()) ?>
      </div>
      <?php if ($comment->comment_approved == '0') : ?>
         <em><?php _e('Your comment is awaiting moderation.') ?></em>
         <br />
      <?php endif; ?>
 
      <div class="comment-meta commentmetadata">
          <a href="<?php echo htmlspecialchars( get_comment_link( $comment->comment_ID ) ) ?>">
              <?php printf(__('%1$s at %2$s'), get_comment_date(),  get_comment_time()) ?>
          </a>
          <?php edit_comment_link(__('(Edit)'),'  ','') ?>
      </div>
 
      <?php comment_text() ?>
 
      <div class="reply">
         <?php comment_reply_link(array_merge( $args, array('depth' => $depth, 'max_depth' => $args['max_depth']))) ?>
      </div>
     </div>
<?php
        }
?>

This code was taken from WordPress Codex, at the same time it’s a default design of the single comment, you can make in this function any changes you want.

In order to implement new “comment theme” you need to configure it. Open comments.php file and find code similar to this:

<ol class="commentlist">
<?php wp_list_comments(); ?>
</ol>

wp_list_comments() function is responsible for displaying comments list. To use our custom function we defined earlier to render comments we need to pass apprioprate argument to wp_list_comments()

<?php
wp_list_comments("callback=mytheme_comment");
?>

From now on each comment will be rendered using mytheme_comment() function. wp_list_comments() accepts more actually more arguments, however i don’t want to duplicate code from Codex here, if you are interested go to WordPress Codex page for more information.