Hi the art style is achieve by a dithering shader: https://godotshaders.com/shader/dither-gradient-shader/
Yes I also feel this one guidance is not clear enough I planed to make some UI and readability improvement but didn't have the time. For the scoring, if you mark something nasty but it isn't, you just don't have the base score. But if you take something nasty away, you'll have penalty on the score. It's my first time designing such score system so i do feel awkward.
About the music, even though im not a good composer I want to make it myself, i just don't have the time... The little tune in the main menu is made by myself.
BTW, this is my ever first game and you are the first one that commented, so thank you!