In this photo the dark theme is achieved not by lighting (which is in any case with a strong contrast), but with the use of a "dark prop", i.e. the item the girl has on her finger. In order to increase the sense of darkness I asked the model to put it on the tongue, so to create a strong picture. In addition, the model has some gothic dress and necklace, to increase the credibility of the image which, in my mind, could be a cover for a music album.