Death Note, Majin Tantei Nougami Neuro Recommendations
Same Animation Studio, OP by same band, deals with the supernatural, overall both are excellent series.
Although Neuro is a lot more light hearted, both animes have supernatural beings interacting (perhaps even manipulating) humans. Both are centered around murder mysteries, although Neuro is episodic (however, there is an overarching unsolved mystery in it too).
Death Note and Neuro both involve very intelligent characters and a great plot. As you progress through either series, a bit of each character is revealed, whether it be good or bad, your bound to like either show. Neuro came from Hell in order to satisfy his appetite for mysteries, and "L" being the best detective in the Death Note world, you're going to be intrigued by either show. Unfortunately because of this, you're going to find yourself wanting to watch more and more. (Also, Both shows have the OP's done by Nightmare.)
Both series are policial/detective series. Both series are really interesting and they encourage you to think about the crimes and to try to resolve them. DN is more violent and dramatic than MTNN and MTNN have a bit of comedy. Both series have a good character design.
Both of this series have dark themes with supernatural mystery counterparts.