When an user visits a post in your blog the plugin registers the time of the visit in a simple cookie. This allows that when the user comes back to the post searching for new comments these could be marked.
To do so the plugin adds the 'unread' CSS class to the comment. You will only have to customize that 'unread' class to mark new comments.
This plugin uses some code from:
Brett Taylor (
http://inner.geek.nz/archives/2005/02/11/wordpress-plugin-smart-unread-comments/)
Utkarsh Kukreti (
http://wordpress.org/extend/plugins/feature-comments/)
For WordPress 3.+:
- Download the last version of the plugin from its page.
- Upload the .zip through the Plugins/Add new/Upload of the WordPress administrator.
- Activate the plugin through the Plugins menu of the WordPress administrator.
For WordPress 2.x (not tested):
- Download the last version of the plugin from its page.
- Unzip the contents of the file in the '/wp-content/plugins/' folder.
- Activate the plugin through the Plugins menu of the WordPress administrator.