A good start, but needs some simple improvements.
The basic idea is really good and simple, which is nice. But you need to dress it up a bit. More color and backgrounds would really give it more excitement, and some sound and/or music would be nice too. Also, sometimes the gray balls just bounce around and don't even go in for a while. This could be helped by making the arena smaller or an AI for the balls. Different difficulties would be a nice addition, too, maybe making the gray balls smarter or hit you harder, go faster, etc.
Also, you should have the background standard, and have a button to turn it off, not on. Again, you did well, but there's lots of relatively little things you could do to improve.