This isn't something I have ever looked up. I just always use the "View New Content" section, or follow posts from the Notification button at top. Here is what I found on the multiple formats:
The Main Website. You can mark each Forum as completely read. If you go to the top of each forum, there is a "Mark this forum as read" button. If there is an easier way to do this from the website, I haven't found it yet.
The Mobile Website. Click on the 4-Square Button in the upper right hand corner. In the new menu that appears, select "Mark All Read."
Tapatalk App, you can select Menu, and "Mark Forum as Read" when you are in the "Latest" tab and it will mark all threads as read.
Forum Runner App, on the Front Page, select Menu and select the "Mark All Read" button.