Source

File: wp-includes/admin-bar.php

function wp_admin_bar_appearance_menu( $wp_admin_bar ) {
	$wp_admin_bar->add_group(
		array(
			'parent' => 'site-name',
			'id'     => 'appearance',
		)
	);

	if ( current_user_can( 'switch_themes' ) ) {
		$wp_admin_bar->add_node(
			array(
				'parent' => 'appearance',
				'id'     => 'themes',
				'title'  => __( 'Themes' ),
				'href'   => admin_url( 'themes.php' ),
			)
		);
	}

	if ( ! current_user_can( 'edit_theme_options' ) ) {
		return;
	}

	if ( current_theme_supports( 'widgets' ) ) {
		$wp_admin_bar->add_node(
			array(
				'parent' => 'appearance',
				'id'     => 'widgets',
				'title'  => __( 'Widgets' ),
				'href'   => admin_url( 'widgets.php' ),
			)
		);
	}

	if ( current_theme_supports( 'menus' ) || current_theme_supports( 'widgets' ) ) {
		$wp_admin_bar->add_node(
			array(
				'parent' => 'appearance',
				'id'     => 'menus',
				'title'  => __( 'Menus' ),
				'href'   => admin_url( 'nav-menus.php' ),
			)
		);
	}

	if ( current_theme_supports( 'custom-background' ) ) {
		$wp_admin_bar->add_node(
			array(
				'parent' => 'appearance',
				'id'     => 'background',
				'title'  => __( 'Background' ),
				'href'   => admin_url( 'themes.php?page=custom-background' ),
				'meta'   => array(
					'class' => 'hide-if-customize',
				),
			)
		);
	}

	if ( current_theme_supports( 'custom-header' ) ) {
		$wp_admin_bar->add_node(
			array(
				'parent' => 'appearance',
				'id'     => 'header',
				'title'  => __( 'Header' ),
				'href'   => admin_url( 'themes.php?page=custom-header' ),
				'meta'   => array(
					'class' => 'hide-if-customize',
				),
			)
		);
	}

}