Comment Highlighter is a plugin which can be used to add different style classes to comments. There are already plugins/hacks/alterations that adds a style to all comments written by the blog owner, adds a different style to every other comment, etc. This plugin takes it a bit further.
Depending on which rules you set up, then Comment Highlighter can add style to comments based on the email of the author (of the comment), the name or the URL. These styles can furhter more be locked to a specific post ID.