This is a simple app coded in JavaScript to look like a ordinary calculator, like these you can find in smarthphones, tablets or PCs.
This is a way the programmer found to learn a new language - in this case, JavaScript - practicing a different technology he learned a little earlier - HTML and CSS. He saw that the result was nice and decided to share the code with his friends.
Not so much time later, he decided to get the calculator and change its color. So, he thought that would be nice if some possible users can change the color of the buttons through its interface. So, he spend more time and coded an JavaScript function to change the color of the calculator according to some previously selected colors.
Actually, he doesn't. Anymore.
He used it a couple of times, but it's a lot easier to use the smarthphone near him. *lol*
He coded the change color's function just for fun.
Have a nice time, or would I say...
Have a nice math. :)