Calculator app challenge hub but Im using simple Js only

I'm making this in mind just to test my Javascript. I'm only using Class and simple algorithms. There's some problem thought and a lot :)

1. I have no idea how to limit the number cause I know it will break the screen and overload everything.
2. I'm only using simple slider to change the theme. I'm dumb about CSS so I don't know how to make the slider into three. I'm trying using pseudo-element on the slide but sadly it didn't accept two pseudo elements on the same div, so I'm kinda lost. Yeah 2 is my limit now. Anyone visiting this know how to make it 3 can you send the links on how to do them?

at first I don't see making this with Class Constructor, before I try using with state concept in react, where I try store information using object and retrieve it back. But I failed. Next time I'm trying to build this using React

yeah thanks for visiting