Creating PDF forms with Flying Saucer was always a pain, especially with CSS2 only support. Frequently used CSS attributes like border-radius are not available, and development speed is slow because it doesn't support HTML previewing of the PDFs. The solution is to treat the form as a background image, and place all text fields and checkboxes on top of it, essentially reducing the need of coding the form with CSS.
A demo version is available in https://pages.choy.in/pdf-editor. You can do basic functions such as positioning new textareas, input boxes, checkboxes, and select a different background form to base off of.