What is JotForm?
JotForm is a free online form builder which helps you create online forms without writing a single line of code. No sign-up required.
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You can actually inject custom CSS code in your form elements, here's a guide that you can refer to: https://www.jotform.com/help/117-How-to-Inject-Custom-CSS-Codes. You can paste your custom CSS code in the Preferences. Or you can paste it on your Form Designer Tool's CSS tab.
However, you need to identify the HTML elements of the matrix table so that you can them as CSS selectors. I hope that helps.
Got this. Thanks for the link!