Sorry for answering so late, I do not check this thread very often any more.
In Settings, navigate to Display -> Advanced -> Style and set Style to "Dark". Then, on the same page, set "Dark mode" from "Default" to "Black".
Unfortunately, the default launcher and other system apps (contacts, phone, gallery, ...) are not yet prepared to pick up a dark system theme. This is Pie after all 🙂. You should be able to find suitable third-party substitutes in most cases, though.