To reproduce run following code



import javafx.application.Application;
import javafx.scene.Parent;
import javafx.scene.Scene;
import javafx.scene.control.Separator;
import javafx.scene.layout.VBox;
import javafx.stage.Stage;

public class SeparatorBorder extends Application {

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

    private Parent getContent() {
        VBox box = new VBox(10);
        Separator separator=new Separator();
        separator.setStyle("-fx-border-color:red; -fx-border-width: 2;");
        box.getChildren().add(separator);
        return box;
    }

    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);
    }
}