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.
At JotForm, we want to make sure that you’re getting the online form builder help that you need. Our friendly customer support team is available 24/7.
We believe that if one user has a question, there could be more users who may have the same question. This is why many of our support forum threads are public and available to be searched and viewed. If you’d like help immediately, feel free to search for a similar question, or submit your question or concern.
I did check your last edited form and did not find any matrix field in form. You seems to have added "Matrix Dynamique" widget in form. Please check the screenshot below how to inject custom css code in widget:
If you want to add custom css code for the "Matrix" field, Please check the screenshot below where css code needs to be added in form:
Hope this helps.
Do get back to us if you have any questions.