Thank you for your feedback!
Regarding the timer: the initial idea, which I also believe was more in line with the theme, was to implement an alarm level that would increase with each fireball spawned; however, due to time constraints, I opted for using a timer, which was easier to implement and required no artwork.
Regarding the darkness: while testing on my pc, the screen was pitch black (on Unity settings, I set the light intensity at 0.02%), but, unfortunately, that's the kind of thing you catch only when the game is played on other machines.
Cheers and, again, thank you for your input.