WP add option to Customizer

class-customizer.php

<?php
/**
 * Customizer additional settings.
 * To retrive the setting use get_theme_mod( 'setting_name' )
 */
class Customizer {
	/**
	 * Class instance.
	 *
	 * @var self
	 */
	private static $instance;

	/**
	 * Get active object instance
	 *
	 * @access public
	 * @static
	 * @return self
	 */
	public static function instance(): self {
		if ( ! self::$instance ) {
			self::$instance = new self();
		}
		return self::$instance;
	}

	/**
	 * Class constructor. Includes constants and init methods.
	 *
	 * @access public
	 * @return void
	 */
	public function __construct() {
		$this->init();
	}

	/**
	 * Run action and filter hooks.
	 *
	 * @access private
	 * @return void
	 */
	private function init() {
		// Setup the Theme Customizer settings and controls.
		add_action( 'customize_register', [ get_called_class(), 'register' ] );
		
		// Optionally we can register assets.
		// add_action( 'customize_preview_init', [ get_called_class(), 'customizer_live_preview' ], 99 );
	}

	/**
	 * Register customizer options.
	 *
	 * @param \WP_Customize_Manager $wp_customize Theme Customizer object.
	 */
	public static function register( \WP_Customize_Manager $wp_customize ) {
		// Add the custom theme options section.

		// Add the header options.
		$wp_customize->add_section(
			'opt-header',
			[
				'title'       => __( 'Site Header', 'textdomain' ),
				'priority'    => 31,
				'capability'  => 'edit_theme_options',
				'description' => __( 'Here you can adjust header elenents.', 'textdomain' ),
			]
		);

		$wp_customize->add_setting(
			'show_extra_bar',
			[
				'capability' => 'edit_theme_options',
				'default'    => 0,
				'transport'  => 'refresh',
			]
		);

		$wp_customize->add_control(
			'show_extra_bar',
			[
				'type'     => 'checkbox',
				'section'  => 'opt-header',
				'priority' => 1,
				'label'    => __( 'Show bar above menu', 'textdomain' ),
			]
		);

	}
	
	**
	 * This outputs the javascript needed to automate the live settings preview.
	 * Also keep in mind that this function isn't necessary unless your settings
	 * are using 'transport'=>'postMessage' instead of the default 'transport'
	 * => 'refresh'
	 *
	 * @return void
	 */
	public static function customizer_live_preview() {
		$source = get_template_directory() . '/build/customizer.js';
		if ( file_exists( $source ) ) {
			wp_enqueue_script(
				'template-customizer',
				get_template_directory_uri() . '/build/customizer.js',
				[],
				filemtime( $source ),
				true
			);
		}
	}
}