Let’s start by looking at the code to achieve styling of author comments prior to Wordpress 2.7. On kremalicious.com I’ve used this code:

<li class="
  <?php
    if ($comment->comment_author_url == "http://www.kremalicious.com")
      echo 'author';
    else echo $oddcomment;
  ?>
  item" id="comment-<?php comment_ID() ?>">
    <em>other comments code</em>
  </li>

So with some php stuff we were able to check for the author name or, as I did it, for the URL of the comment author. If one of these were detected Wordpress added a new class ‘author’ to the <li> tag which we were able to style by adding a li.author to our css file:

li.author {
  /* css comes in here */
}

But with Wordpress 2.7 these steps are needless because of the new function <?php wp_list_comments(); ?> which adds a class on author comments for itself!

If a comment from the author of an article is posted under this article, Wordpress automatically adds the class ‘bypostauthor’ to the surrounding <li> tag. So all you have to do is adding a css style of li.bypostauthor to your css file or just renaming your old li.author class or whatever you used for this:

li.bypostauthor {
  /* css comes in here */
}

And that’s it for adding a different style to comments from the article author. Just add some css and there you go. Wonderful!

#Even more

Wordpress also has a special class for registered users of your site so you’re able to style their comments as well. For this just use the class ‘byuser’:

li.byuser {
  /* css comes in here */
}

All the various classes Wordpress adds to comments are listed in the Codex page for enhanced comments display. And here’s a very nice grahical overview about everything Wordpress 2.7 adds to comments.