You will need to install a user script manager extension to install this script.
Occasionally, if you go to User CP after logging in, the 'last visited' time might get reset to the current time and there will be nothing in New Subscribed Threads list, so it's easy to lost track of the new posts.
This script attempts to mend this a little: it cannot bring the list back, but it reads and remembers the 'You last visited' time displayed right after login and dims posts older than that time (by reducing opacity). It makes it easier e.g. to go to List Subscriptions, click links to Last Post from there and see up from where the actually new posts begin.
A menu item is also added to the Thread Tools menu that allows to disable this script functionality for the current session (until new login).
In order for the script to work, it must be activated before logging in!
Experimental, for now tested with Violentmonkey only.