There's an option to do so within the FAB, which I think is only in the alpha release for now.
It doesnt seem to work like i would expect it to. When i tap it i would expect to get a list of undread only from say the past 24h, but nope. It does bring unread posts to the top but still has read posts under them. Feeder works the way i think thunder should for unread only view.
Interestingly enough, I prefer the current way of hiding read posts because I like to know which posts are still active periodically.