how to use custom css

If you have already a custom CSS code. You can inject it to your form by following the guide on this link https://www.jotform.com/help/117-How-to-Inject-Custom-CSS-Codes.

Here are CSS code samples: https://www.jotform.com/help/117-How-to-Inject-Custom-CSS-Codes

Guide on how to inject CSS code to your widget that has CSS field https://www.jotform.com/help/428-How-to-Inject-CSS-Codes-to-Widgets

Please take note that Card Forms (Single Question in One Page) doesn't have a custom CSS field and doesn't support modifying the form using CSS by any means.

Let us know if I have misunderstood your question.