Normally after a customer submits a form, they will receive a message that notifies that the form has been submitted successfully. In this tutorial, we'll show you how to customize this success message in Blue Form Builder, a powerful Magento 2 form builder extension.
https://www.magezon.com/magento-2-form-builder.html

Open the edit page of a form. Then open Settings > Success Message where you can customize the success message of the form.
- Enter the message content in the WYSIWYG editor. Please note that you can insert variables here so that your message will be dynamic.
- Header Content: enter the content in the header.
- Footer Content: enter the content of the footer. The footer content only appears when you enable redirecting all customers after submission.
https://blog.magezon.com/redirect-customers-blue-form-builder/
- Style: choose style for the message of 2 options available. If you choose Style1, you can customize text color, background color and border color between the header and main content. If you choose Style 2, you can customize the circle color.