hiya mlg
once u click on a thread you cannot mark it unread but you can save them to your favorites. At the bottom of the page of the thread you will see this:
Home
Subscribe topic Bookmark topic FAQ †Delete all board cookies †All times are UTC - 6 hours [ DST ]
When you click Subscribe, you will receive automatic emails when anyone posts to that thread.
What you prolly want though is to click the Bookmark.
This will save it to your favorites, to access your bookmarks click the UCP tab at the top of any page and then on the left menu click on Manage Bookmarks. You can then see which threads you want to reply too and afterwards you can go back and remove the bookmarks if you like.
You can manage your subscriptions from that UCP page as well.
Hope this helps
God Bless you sister