JavaFX - Why there is a line under every Control element and how to remove it?

I noticed that every element which inherits from the Control class has an underscore at the bottom. You can see it in the picture:

When I put a focus on any of items, the line disappears. Why does it happen and how can I get rid of that?

Code:

Main.java

package sample;

import javafx.application.Application;
import javafx.fxml.FXMLLoader;
import javafx.scene.Parent;
import javafx.scene.Scene;
import javafx.stage.Stage;

public class Main extends Application {

    @Override
    public void start(Stage primaryStage) throws Exception{
        Parent root = FXMLLoader.load(getClass().getResource("sample.fxml"));
        primaryStage.setTitle("Hello World");
        primaryStage.setScene(new Scene(root, 300, 275));
        primaryStage.show();
    }


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

sample.fxml

<?import javafx.scene.layout.GridPane?>
<?import javafx.scene.control.Button?>
<?import javafx.scene.control.ProgressBar?>
<?import javafx.scene.layout.HBox?>
<?import javafx.scene.control.ComboBox?>
<?import java.lang.String?>
<?import javafx.collections.FXCollections?>

<GridPane stylesheets="@sample.css"
          xmlns:fx="http://javafx.com/fxml"
          alignment="center">
    <HBox>
        <ProgressBar minWidth="100" progress="0.3"/>
        <Button text="Button"/>
        <Button text="Button"/>
        <Button text="Button"/>
        <ComboBox>
            <items>
                <FXCollections fx:factory="observableArrayList">
                    <String fx:value="String" />
                    <String fx:value="String" />
                    <String fx:value="String" />
                </FXCollections>
            </items>
        </ComboBox>
    </HBox>
</GridPane>

sample.css

.root {
-fx-background-color: gray;
}