For most complex and sophisticated UI requirements, wouldn't it be easier for designers to use a tool to design their UI with a WYSIWYG interface, without writing any code, and then load the result (FXML file) into their JavaFX application logic?

Therefore, you need JavaFX Scene Builder; it is a visual layout tool that allows you to easily lay out your UI controls so that you can quickly prototype your application with effects and animations. Scene Builder (version 2.0 upwards) is the compatible version for JavaFX 8.

At any time during the creation of your project, you can preview your work to check its real appearance before deploying it.

It is open source and therefore it integrates with most IDEs, but more tightly with the NetBeans IDE. It is also a cross-platform, self-contained application that runs on most platforms.

In addition to supporting CSS, it allows you to easily apply custom theming to your prototype.