Hello,
My name is Akos, I'm a professional web developer and I also have a passion for calculators. Have done much more complex ones.
I've checked out both the links you've attached. What you basically need is the calculation part, right, which is coded in javascript on both sites. I actually usually use jQuery which should be loaded on most of wordpress sites already, so that won't be a problem.
What do you exactly mean by being able to customize the CSS? Can you edit an existing CSS code?
Would love to have a short chat to clarify things and I would be able to start immediately.
Kind regards
Akos