This was a cute version of Sudoku. I enjoyed the art and it fit the Halloween theme well. The background was nice, adding color and character without distracting from the puzzle. The color contrast between the background, grid, and numbers felt good to me, and I could clearly see the numbers. The different expressions on the character were fun. I liked the location of her, allowing her to be clearly visible without being in the way of the active puzzle. I also liked that you gave different options for the background music. All three fit the Halloween theme well, and it was nice being able to go between them when I wanted something a little different.
You did a good job with making it mostly playable with just keyboard, as well as completely playable with just mouse or a combination of both. I think it would be nice if you included a way to change the difficulty with the keyboard. You could possibly use the “+” and “-” keys to increase or decrease the difficulty.