I don't know if I'm missing it but I have looked for a 'mark all as read' button but can't find one. I know you can go into the individual sub-forums and mark those as read but there used to be one on the front page where you could select all the forums as having been read.
Having the little orange 'new' signs on forums I don't read is playing havoc with my OCD. It's like having unread email numbers on my phone
Belay my last. Edited to add that it's in the drop down menu of the Forums tag at the top of the main page....if anyone has the same mental 'challenges' as I.