CSS Import Error with Java 11

[marcel Austenfeld]

For my application I apply a custom stylesheet for the JavafX components, e.g., 
to switch from a light to a dark theme to colorize, e.g., the SceneBuilder and
other JavaFX components I use.
 
So at startup I load and apply a custom stylesheet with: 
javafx.application.Application.setUserAgentStylesheet(myPath/my.css);
 
In the file my.css I import the default modena.css with:
 
@import "com/sun/javafx/scene/control/skin/modena/modena.css";
 
Then in my.css I overwrite some styles.
 
This works fine in Java 8 but fails with Java 11 with the following message:
 
WARNING: resolveRuntimeImport cannot resolve: 
com/sun/javafx/scene/control/skin/modena/modena.css
Okt. 10, 2018 9:45:36 VORM. com.sun.javafx.css.StyleManager 
loadStylesheetUnPrivileged
INFO: Could not find stylesheet: 
bundleentry://273.fwk1678623942/javafx/com/sun/javafx/scene/control/skin/modena/modena.css
Okt. 10, 2018 9:45:36 VORM. javafx.css.CssParser handleImport
 
Is this not valid anymore in Java 11? Are there any workarounds to apply a 
theme in Java 11?
 
Thanks in advance for any answer.