Hi … Wix does not support CSS.
When you create or code something you build the interface using Wix elements by dragging and dropping them on the page.
Using backend code is doable but a bit complex for novice coder.
Since it seels like you may already have the ‘formula’ to make your calculations, you want to consider creating the calculation code right on the page itself (instead of the backend).
Here is a tutorial that shows how to create simple math calculations by building a small form on a Wix page: https://support.totallycodable.com/en/article/create-simple-math-calculations-or-instant-online-quote-using-wix-code