Can i suggest to add some CSS styles?

[Benjaminmq2013]

I saw your calculator and it's really cool, and i saw that it hasn't CSS.
So i worked on it, and i want to show you this and see what do you think?
let me know if you want to add them. I send you greetings

note: It did not interfere or modify the JS

i can make a pull request if you want to check the code