Overall a fun stylized platformer with a cool mechanic and a lot of potential for further development and exploration. I hope you keep working on this as I think it has the potential to do some really cool things.
I did find a few bugs. The credits screen doesn't scale correctly and is just stuck to the right side of the screen. Also, when coming out of the dark mode, if the character hits a wall they can sometimes stay in the wall jump pose and just kinda slide along the ground.
I suggest adding some particle FX to the character's hair to simulate smoke coming off of the fire. I also suggest adding some particle FX to the feet for movement would help give the game more juice and help it to feel better overall. I also suggest making a more linear level 1, I was confused that I needed to find the end portal, something more linear that maybe established a story could be nice if you wanted to keep working on this.