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.
You should be able to change the form warning color and design by using the Form Designer Tool.
In the screenshot below, open the "Color Scheme", select "Simulate State error" and open the settings. You can change the color there and the style of the form warning box.
You can also make advance styling on the form warning message by using custom CSS code. Here are some list of custom CSS code that you can use as a reference: http://www.jotform.com/help/75-Customize-your-Form-using-Custom-CSS-Codes
I hope that helps.