The modena css applies some nice color derivations from -fx-base. The effect is that all controls' look is similar, in a rather "subdued" coloring. Writers of controls want to copy that effect, but have to guarantee that their work will still provide "nice" results with future JDKs, too. As a consequence, it is essential to have a public reference that describes how to obtain this effect, without voiding "visual compatibility" with future JDKs.

For example, one trick often applied is that custom control providers apply "-fx-base", which effectively makes their custom control look quite nicely and adequate to modena.css. But there is no written guarantee in the JDK that "-fx-base" will even exist, or actually have that effect in all future versions of JavaFX. Even worse, many already derived colours do exist in modena.css but cannot be used as there is no guarantee that the particular reference will work in times past-modena.

Moreover, not all controls use this trick. For example, the colour palette of charts is fixed currently, so one cannot simply modify -fx-base to get bar charts with a custom colour but having the same "subdued" effect applied.

To sum up, there must be a publicly available description how to obtain that effect in a write-once run anywhere cross-version cross-vendor way.