If you have a WordPress powered blog with lot of comments, it might be good idea to somehow highlight comments posted by Administrator. It looks better and it's much easier to follow comments.
1. To do so, open the style.css of theme you use and place anywhere this code:
/* CSS styles for author comments */
background-color: #FFFFCC !important;
Of course, instead of "FFFFCC" you can use any code for any color - there are some of them: link
Note: This specific CSS class "li.bypostauthor" is automatically added by WP to author comments. For registered users WP adds "li.byuser" CSS class.