On the previus versions (5.x) of UBB trehads, when you open the "Last 24 Hours" or "Last 48 Hours", you will see a difference on threads you already read and the new ones to you. Normally the new ones are in BOLD and the read ones are in NORMAL character.
Now, using the 6.5.1.1 it is not happen any more.
I always used the "desert-classic" style only but now, all threads list as the same and is impossible to know what you read, what you not.

Do you have any solution for it on your forums?
I try few styles and all have the same behavior on this issue.

I would like to have some way to show the user what he already read and what not.

Thank you

Check out the link just above your post contained in "A map of all the CSS classes can be found here."