On Windows, one may end up with a mix of '/' and '\' in an url, and get errors such as:
WARNING: Resource "jar:file:/C:/wksfx/sb20/SceneBuilderKit/dist/SceneBuilderKit.jar!/com/oracle/javafx/scenebuilder/kit/editor/panel/inspector/editors/..\..\..\css\Theme.css" not found.

(see the attached fxml that produced this warning).
We should use '/' everywhere to handle the paths more easily and avoid all these separator issues.