Another theme could be the exploration of the subconscious. The dark often represents the hidden parts of our minds in literature. The 'bonus shadow' could be related to something that lurks in the subconscious, like a repressed memory or a hidden desire, and the story could be about the journey of uncovering it.
I think this short story might be about a transformation that occurs at dark. The 'bonus shadow' could be something mysterious that appears as a result of this change. Maybe it's a story of the supernatural or a psychological exploration of how a character's perception changes in the dark.
Isolation can also be a theme. A person who is isolated from society, either by choice or by circumstances, can have a dark shadow over them. They may feel lonely, and this sense of being alone can create a negative atmosphere around them, much like a dark shadow.
One possible theme could be duality. Just as the sun and shadow are two opposing elements, the story might explore the two - sided nature of human beings or life in general.
One possible theme could be transformation. The girl's change from a human to a fish is a major transformation that can represent change in general in life.
Fear is a likely theme. The darkness of the room can symbolize the unknown which often induces fear in people.
One possible theme could be isolation. The dancer is in the dark, which might suggest she is alone. Another theme could be self - expression. Through dancing, she is expressing something about herself.
Another theme might be the power of reflection. Not just in the literal sense of looking in the mirror, but reflecting on one's actions and choices. In a dark mirror context, this reflection could be a source of pain or regret. For example, a character might see in the mirror all the wrong decisions they've made in their life and how it has led them to a dark place. It could also touch on the theme of isolation. If the mirror shows a world that is different from the real one, the character might feel isolated from others who don't experience the same distorted view.
One possible theme could be fear. Since it involves 'the dark ones', fear of the unknown or the malevolent is likely.
Fear could be a theme. The darkness in which the murder occurs is often associated with fear. The characters in the story might be driven by fear, either the fear of the unknown killer or the fear of being wrongly accused. Also, the fear of the dark can heighten the tension in the story.
One possible plotline could be that his friends or allies have to take care of him in his baby form. They might struggle with things like feeding him, changing his diapers, and keeping him out of trouble.