When in threaded view, a collapsed thread with unread messages is currently marked by underlining the subject of the top message. From an accessibility point of view this is not enough. The underline is barely noticeable in a list of messages/threads. Specially if they contain single, unread messages that stand out in the usual bold font. I am short sighted, and had to disable threaded view again after I missed messages because of this barely visible way of flagging threads with unread messages. So my suggestion would be to either use bold & underline in these cases, or a different color.