Above answer only explains how to highlight comments made by the author. If you want the ability to highlight every comment made on your blog I highly recommend you to follow this tutorials from Chris Coyier on CSS-Tricks: <a href="http://css-tricks.com/curating-comments-threads/">Curating Comments Threads</a>

In order to highlight comments on your WP blog you'll need to edit your CSS and comments.php files with fairly simple code.

Here's what you'll need to add to your CSS file (towards the bottom):

.authcomment {
background-color: #B3FFCC !important;
}

Your comments.php file should've contained a line that looked similar to this:
” id=”comment…

Now edit it to look like this:
/* Only use the authcomment class from style.css if the user_id is 1 (admin) */
if (1 == $comment->user_id)
$oddcomment = “authcomment”;
echo $oddcomment;
?>” id=”comment…

Source