To reproduce run following code
Unexpected: gray background behind border


import javafx.application.Application;
import javafx.scene.Parent;
import javafx.scene.Scene;
import javafx.scene.control.*;
import javafx.scene.control.Label;
import javafx.scene.control.TitledPane;
import javafx.scene.layout.VBox;
import javafx.scene.paint.Color;
import javafx.scene.shape.Rectangle;
import javafx.stage.Stage;

public class Accordions extends Application {

    public static void main(String[] args) {
        launch(args);
    }

    private Parent getContent() {
         VBox list = new VBox(10);

        TitledPane pane1 = new TitledPane();
        pane1.setText("title 1\nLong text long text");
        pane1.setContent(new Rectangle(100, 40, Color.SKYBLUE));
        TitledPane pane2 = new TitledPane();
        pane2.setText("title 2\nLong text long text");
        pane2.setContent(new Rectangle(100, 40, Color.BLUEVIOLET));
        Accordion acc = new Accordion();
        acc.getPanes().addAll(pane1, pane2);
        acc.setExpandedPane(pane2);
        acc.setStyle("-fx-border-width:5;-fx-border-color: green;-fx-border-style: dashed;");
        list.getChildren().add(acc);
        return list;
    }

    public void start(Stage stage) {
        stage.setX(100);
        stage.setY(100);
        stage.setWidth(300);
        stage.setHeight(300);
        Scene scene = new Scene(getContent());
        stage.setScene(scene);
        stage.show();
        
    }
}