@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!
Didn't work postcount for sub category & didn't work preview
In v.0.7.1 & v.0.7.2 correctly count posts in old structure of sub category.
But if create new category & create post in sub category - counter didn't "see" posts in subcategory & didn't preview posts in main page.
Bug report in git - https://github.com/NodeBB/NodeBB/issues/3368