I spent some serious time on this, and finally found out that since Version 2.7, WordPress outputs a CSS class that you can use (see WordPress excerpt below). The class is "byauthor", and adding this line to my style.css did the trick in setting the author's comments apart:
/* CSS styles for author comments */
background-color: #d4e5b1 !important;
No need to edit the PHP, just using and styling the WordPress-created class did the trick.
The new comments loop automatically puts every comment into a list (li) tag, and threads them as well. It adds classes on all HTML list tags that surround every comment in this fashion:
* comment, trackback, pingback classes get added depending on the type of the comment.
* byuser gets added if the comment is by a registered user of the site.
* comment-author-authorname gets added for specific registered users.
* bypostauthor gets added if the comment is by the author of the post the comment is attached to.
* odd and even classes are added to odd and even numbered comments
* alt is added to every other comment
* thread-odd, thread-even, and thread-alt classes are the same as the odd/even/alt classes, but these only apply to the top level of each set of comments and replies
* depth-1 is added to the top level comments, depth-2 to the next level, and so on.
* children class is added to all threaded comments below the parent list
I leave it to someone else to figure out the multi-admin problem, but these links might provide some clues: