I want to add custom CSS statements to style my calculator. Where should I add them?

If you use Visual Composer you should click on Page Settings button in the upper right corner of Visual Composer's interface on relevant Page Edit Form:

The Page Settings dialog will be displayed.
Please enter Custom CSS statements in the Custom CSS settings field.

If you do not have Visual Composer, please add the statements to your main CSS file or, alternatively, use third party plugin like: https://wordpress.org/plugins/simple-custom-css/.