I think this is a pretty cool and unique idea for a core gameplay mechanic! The sprites also look amazing, and I really like the music you used in the game.
Gameplay-wise though, I had a pretty game-breaking bug that I kept running into (at least I assume it's a bug). After attacking for a little while, I would abruptly become unable to attack at all by left or right-clicking. This lasted between rooms, and the only way I found to fix it was resetting the game from the start. This happened consistently; I saw it 4 times in as many attempts just by attacking enough. It didn't seem to have anything to do with the light/dark gauge, since it was always roughly balanced when I froze up.
Apart from that bug, I wish there was a little more done with the light vs dark mechanic. As far as I can tell, there was no real reason to use one over the other besides managing the gauge, so the mechanic boiled down to "use left click to attack until the gauge is too far one way, then use right click until it's balanced again". It would've been nice to see a more dramatic difference between the two, like if the dark magic was a melee-type attack to force you to get up close or something.
This is definitely an idea worth working on more; I look forward to seeing how it looks if you do!