It's a simple platformer, using illumination in two facets: 1. To mess with visibility, 2. To get by ghosts.
For those of you who are also struggling, this game's dev has revealed the secret to me.
This game's controls assume the AZERTY layout, which is used in France and Belgium. Therefore, the real brightness keys to us common QWERTY users are actually Q and W.
When you play the game with this knowledge, it makes much more sense. Being bright gives you more visibility, and freezes white ghosts, but you expend more life as an upkeep to do so. Being dim gives you blindness, but makes you invisible to black ghosts and costs no life as an upkeep.
However, I believe the game does not have enough to quantify a full experience. What is made here is competently made, where the complaint is stemming from a cultural issue.
You will have to consider these things in the future. Your common audience will be on QWERTY, which is going to take you out of contention from accessibility alone. Knowing, this however, will make you that much stronger if you come back