@julian, I think I fixed this issue. I rolled back to a snapshot before I installed NodeBB and re-cloned the 0.5.1 branch and it installed without an issue and I have sweet, glorious CSS!
[Nodebb website] Cannot Mark All posts as read
From the unread posts page (on this website), I can no longer use the Mark As Read "All" functionality. I'm guessing this is a development bug?
Thanks for reporting fixed in https://github.com/NodeBB/NodeBB/commit/aa30973f0a628bd2de91741c483b2a23a28e913d