JotForm is a free online form builder which helps you create online forms without writing a single line of code. No sign-up required.
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.
You can do that by using injected custom CSS code on your form.
Check this user guide http://www.jotform.com/help/75-Customize-your-Form-using-Custom-CSS-Codes
There are some rounded corner form templates in the JotForm Template gallery that you can already use.
Inform us if you need further assistance.