A DESCRIPTION OF THE PROBLEM :
According to the JavaFX CSS Specification, Colors are the only properties that support lookup to custom properties.

.root {
  --primary--background-color: black;
  --secondary-background-color: rgb(0, 0, 0);
}

It would benefit a lot having support to define custom properties for more than just colors, but also for Size and Font.

.root {
  --primary-font-family: "Segoe UI Semibold";
  --secondary-font-family: "Roboto";
  --primary-padding: 10px;
  --secondary-padding: 5px;
}

It would preclude any use of a preprocessor like SASS to do this.