Topic and Forum listings are color-coded to help you identify topics or forums with content that you have not yet read. If you wish to mark something as 'read' without actually opening it, you have a few ways to do so.
To Mark a Forum as 'Read' from the Home Page:
In this example, the top two forums ("Beverage & Libations" and "Cookbooks & References") have no unread content. The "Cooking" forum has unread content. To mark the content as 'read' without reading it, click on the icon circled in red.
To Mark a Forum as 'Read' from its topic list:
Near the upper right corner of a page is the label "Mark forum as read". Click it.
To Mark a Single Topic as 'Read' from a topic listing:
In this case the topic listing is from the "View New Content" activity stream.