How to access Modena resources?

[Wolfgang Zitzelsberger]

For some custom controls we extend the Modena theme by providing an 
additional CSS file. In this file we have references to some image files 
of Modena just like below.

.details-dialog .message-icon{
  -fx-image: 
url("/com/sun/javafx/scene/control/skin/modena/dialog-information.png");
}
However, because the package is not open this does not work in Java 9 - 
see also 
https://stackoverflow.com/questions/45414557/modena-css-resource-loading-fails-on-java-9

- Wolfgang