After a good sleep, I updated the game so that it doesn't just do addition but multiplication, subtraction and division. The latter two needed to be implemented such that there were no negative numbers and only integer numbers.
I then updated the appearance so it's a little neater and less of a prototype as well as swapping out some sounds for slightly more appropriate ones. Some of the appearance update was adjusting the UI so that it scales better on mobile screens.
I also added some buttons after the game finishes so that the keyboard isn't required to navigate.