If (what I'm not sure) understand your question correctly, you want to add an extra class if the comment was posted by the node author.
As explained in Bartik documentation:
/* - $classes: String of classes that can be used to style contextually through
* CSS. It can be manipulated through the variable $classes_array from
* preprocess functions. The default values can be one or more of the following:
* - comment: The current template type, i.e., "theming hook".
* - comment-by-anonymous: Comment by an unregistered user.
* - comment-by-node-author: Comment by the author of the parent node.
There are already specific classes for comments posted :
- by node author :
- by anonymous :
And you can see that in your node markup :
<div class="comment comment-by-node-author clearfix" about="/comment/2#comment-2" typeof="sioc:Post sioct:Comment">
/* ... */
<div class="comment comment-by-anonymous clearfix" about="/comment/3#comment-3" typeof="sioc:Post sioct:Comment">