Check out example codes for "add css to javafx fxml". It will help you in understanding the concepts better.

Code Example 1

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

Code Example 2

<StackPane maxHeight="-Infinity" maxWidth="-Infinity" minHeight="-Infinity" minWidth="-Infinity" prefHeight="400.0" prefWidth="600.0" xmlns="http://javafx.com/javafx/8" xmlns:fx="http://javafx.com/fxml/1" fx:controller="app.fxml.SettingsController" stylesheets="@app/cssfilepath.css">

......
.....
.....
</StackPane>

Code Example 3

.sun-button {
  -fx-graphic: url('./icons/sun.png');
}

Code Example 4

package application.view;

import javafx.fxml.FXML;
import javafx.scene.control.Button;

public class RootLayoutController {


    @FXML
    Button sunButton;

    @FXML
    public void handleSunButtonClick() {
        System.out.println( "Button clicked");
    }
}

Code Example 5

<?xml version="1.0" encoding="UTF-8"?>

<?import java.lang.*?>
<?import javafx.scene.control.*?>
<?import javafx.scene.layout.*?>
<?import javafx.scene.layout.AnchorPane?>

<AnchorPane xmlns="http://javafx.com/javafx/8" xmlns:fx="http://javafx.com/fxml/1" fx:controller="application.view.RootLayoutController">
   <children>
      <Pane layoutX="0.0" layoutY="0.0" prefHeight="200.0" prefWidth="200.0">
         <children>
            <Button fx:id="sunButton" layoutX="74.0" layoutY="88.0" mnemonicParsing="false" onAction="#handleSunButtonClick" styleClass="sun-button" stylesheets="@../css/toolbar.css" text="Button" />
         </children>
      </Pane>
   </children>
</AnchorPane>

Code Example 6

package application;

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


public class Main extends Application {
    @Override
    public void start(Stage primaryStage) {
        try {

            FXMLLoader loader = new FXMLLoader(Main.class.getResource("view/RootLayout.fxml"));
            AnchorPane rootLayout = (AnchorPane) loader.load();

            Scene scene = new Scene(rootLayout, 400, 400);
            scene.getStylesheets().add(getClass().getResource("css/application.css").toExternalForm());

            primaryStage.setScene(scene);
            primaryStage.show();

        } catch(Exception e) {
            e.printStackTrace();
        }
    }

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

Learn ReactJs, React Native from akashmittal.com