.group .rect {
  -fx-fill: #550000;  
}

.group{
  -fx-fill: #550000;  
}

package proba;

import javafx.application.Application;
import javafx.scene.*;
import javafx.scene.Scene;
import javafx.scene.paint.Color;
import javafx.scene.shape.Rectangle;
import javafx.stage.Stage;

public class Prob extends Application {
    
    public static void main(String[] args) {
        launch(args);
    }
    
    @Override
    public void start(Stage stage) {
       Group root = new Group();
       Scene scene = new Scene(root, 500, 500, Color.BLACK);
       
       scene.getStylesheets().add("proba/style.css");
       root.getStyleClass().add("group");
       
       Rectangle r = new Rectangle(25,25,250,250);
       r.getStyleClass().add("rect");
       
       
       root.getChildren().add(r);
       
       
       stage.setScene(scene);
       stage.show();
   }   
}