While viewing DU in Standard or Classic mode (but not in Mobile view), if you look under an opening post on the right side you can see the options as links - Always highlight: 10 newest replies | Replies posted after I mark a forum.
If 10 newest replies is selected, each thread should show the last ten replies as yellow unread icons and the older ones as white read icons.
If you select Replies posted after I mark a forum then any replies made in that forum after you hit the Mark as read will have the yellow unread icon and the ones made prior to you marking the forum will use the white read icons.
Unfortunately you can't track new posts in individual threads - only by each forum/group. Users on Mobile view also have the issue of not being able to see the links to select how the mark function will work for them. Here is the web address to toggle between the two options:
(NOTE: do not click this link unless you want to change your Mark as Read setting!)
https://www.democraticunderground.com/?com=thread&address=125614163&togglemark=1#toc
That thread has over ten replies, so it should be easy to see how the mark function is set.
The Mark as read button is near the top of each forum page listing the threads. For example:
https://www.democraticunderground.com/?com=forum&id=1256
That''s what we have, you'll have to play with it to decide if you find it useful or not.