Congratulations on finishing the challenge!
Your code is pretty easy to read, that's quite an achievement already. And README
file is very helpful and full of appreciation.
For your question about float point number, you can do something like:
Math.round(1000000000000 * eval(display.value)) / 1000000000000
The number of zero is up to how many decimal places you want to have.
And you might want to have this line outside of functions.
const display = document.getElementById('display');
Declare display
as a global variable, so that it can be used by every function.
And your solution of Theme Toggle is actually interesting. But that way, the number label doesn't corporate with the theme. I recommend you to have a look at this code, it's how to build a toggle button that functions as it's supposed to be.