Oh my, it's dark. Dark and bleak. The Joker origin story, told plausibly and pretty much sympathetically. The world made him the way he is. Turns out he doesn't mind killing at all though, so a monster is unleashed. J Phoenix literally acts his shirt off.
But honestly, I can't see who this film is made for; maybe Batman completists who want every little dark corner of the Batman world totally filled in. The film is well-acted, reasonably well-scripted, and mostly logical in a comic book kind of way, but if you want to know whether you'll ENJOY it, that is an entirely different bucket of face-paint.
Borrows shamelessly from The King of Comedy amongst others. Dark and bleak, dark and bleak, are the lands where the Joker lives. His hair is green and his hands are... RED with blood.