Art-style is great! Very, very short though, and a bit one dimensional. I've done those reaction time online tests before and I'm extremely slow with those so was worried that I'd get smoked with this one, but didn't have any trouble with that.
I think it could be nice to communicate back to the player how quickly they reacted, whether in raw ms of reaction time or something else.
Made in one day is crazy though, great work for doing it in such a short amount of time!