To reproduce run following code



import javafx.application.Application;
import javafx.scene.Parent;
import javafx.scene.Scene;
import javafx.scene.control.Label;
import javafx.scene.control.TitledPane;
import javafx.scene.layout.VBox;
import javafx.scene.text.Text;
import javafx.stage.Stage;

public class TitledPaneBorderBackground extends Application {

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

    private Parent getContent() {
        VBox list = new VBox(10);
        TitledPane outerPane = new TitledPane();
        outerPane.setTitle(new Label("OuterPane label"));
        Text text = new Text("Content Content\n\nContent");


        outerPane.setContent(text);
        outerPane.setStyle("-fx-background-color: green; "
                + "-fx-shape: \"M 50 50 L 150 50 L 100 150 Z\";"
                + "-fx-scale-shape: false;");
        list.getChildren().add(outerPane);
        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.setVisible(true);
        scene.getStylesheets().add("/style.css");
    }
}