Our website has nothing to do with this.  The thread color is strictly a function of your browser and you browser history.  When a link is displayed, your browser looks in history to see if you have been there before.  If you have been there before it looks at dates to see if the link has been updated since your last visit.  If your browsing history has been cleared, all links will show as unread.  If you have your browser set to clear history, then the links will not change color.