Nice work. The opening theme feels quite calm. It's a nice theme, but it didn't really give me the feeling that I was about to play something scary. This may have been intentional, I don't know. Sometimes it can indeed be nice to give a false sense of security before you drop the player into the dark.
Rolling into the main theme next, I felt things were picking up speed a bit. I quite like the drums and flute. Especially the second drum set is quite nice. Again though, it didn't feel particularly scary to me.
Stage 1 is next. This definitely seems a bit darker. Here I think the timbres you chose make it quite a bit more creepy already. I think the juxtaposition of the kind of happy melody in major key and the darker baseline definitely made me a little uneasy. I was expecting the track to slowly get more extreme over time and it did a little, but then the track was over. I was hoping for a bit more dissonance, or some unusual sounds. Still a good track though!
The boss fight definitely sounds a bit darker still. I do like that the soundtrack slowly gets darker and darker as it progresses. It does make you wonder what comes next. I also like the tremolo you have here on the strings.
Overall, still a very cool theme. Favorite is definitely the boss theme.