JavaFX 2.0 :: Styling Of Controls Doesn't Work programmatically

I have a stylesheet (mystylesheet.css) with following entry:
 
.flowpane{
    -fx-background-color: rgb(0,0,0);
}
 
In then start-method of my application I execute:
 
       StackPane root = new FlowPane();
        Scene scene = new Scene(root,primaryScreenBounds.getWidth(),primaryScreenBounds.getHeight());
        scene.getStylesheets().add(css.getFile());
        root.applyCss();

No FlowPane gets black. But if I do that in SceneBuilder and add the stylesheet to root than after loading Fxml-file the FlowPane gets black.

Another approach:

        StackPane root = new FlowPane();
        Scene scene = new Scene(root,primaryScreenBounds.getWidth(),primaryScreenBounds.getHeight());
        scene.getStylesheets().add(css.getFile());
        root.getStyleSheets().add(css.getFile());
        root.applyCss();

Nothing happens.

        StackPane root = new FlowPane();
        Scene scene = new Scene(root,primaryScreenBounds.getWidth(),primaryScreenBounds.getHeight());
        scene.getStylesheets().add(css.getFile());
        root.getStyleSheets().add(css.getFile());
        root.getStyleClass().add("flowpanel"); 
        root.applyCss();

Doesn't work too;
 
but root.setStyle("-fx-background-color: rgb(0,0,0) works. But I need to style my application by css-files.