JavaScript Practice, not really mobile friendly xD

Hello, with this project my goal was to practice JS. As I was building this project I just wanted to make it look good enough on desktop so that I can focus on writing JS and now I can't make it really responsive. I did try though, but couldn't really make it work. It would be easier to start from scratch and keep in mind responsiveness.

I would like to know if it's possible to allow for input fields to accept only numbers (0-9) by users, I did google something like "input field restricted to numbers" or "input validation" but could not find a good solution.