Is it possible to “reset” threads that have been read? Basically, the opposite of “mark all as read”. After being here a few years, I’d like to go back through the forum to re-read. What I’m looking for is to make all the threads white again, so I when I read them again they will turn grey. 

At the moment, and as far as I can tell, no - there isn’t. It’s possible that one of the more expert code people might know some sort of fix, but in more years than I care to admit of lurking on internet forums I can’t recall being able to ‘mark all unread’.