This game is really good.
I really love what you did with the shiny background. The bullet when you shoot is also phenomenal as well. Suggestion for graphics: Even though I really like the background, it would be better if the other graphics in the game is of the same type. Making 2D graphics on a 3D-ish background is okay, but I think you can remove the black border in your game sprites (the player and the zombie). Also maybe make the score and the number of bullets left box bigger.
Audio: I really like the theme song and the sound effect when you shoot. But maybe add a creepy background audio when you play the game and raise the volume of the shooting sound would help you make the game more scary. (I assume that this is a horor-ish game as it is dark. If it is not, just raise the volume and your game might be better)
Theme: The theme in a way fits the theme. However, I would like more explanation for why the light zone is smaller, the more kills you get. The theme is also the more you HAVE so maybe you could also add some additional function to make it fit the game better. Having more kills is a little bit clichey.
Overall, really good game, I really love it when it gets darker.