Using Customizer Blank Slate with your theme

functions-add-blank-slate-param.php

<?php

/**
 * Append the customizer url, in Admin Bar, with the parameters that triggers Customizer Blank Slate. 
 *
 * @author soderlind
 * @version 1.0.0
 * @param   string    $url     The complete admin area URL including scheme and path.
 * @param   string    $path    Path relative to the admin area URL. Blank string if no path is specified.
 * @param   int|null  $blog_id Site ID, or null for the current site.
 */
function add_admin_bar_customizer_blank_slate_url_param( $url, $path, $blog_id ) {
	if ( false !== strpos( $path, 'customize.php' ) ) {
		$url = add_query_arg( 'customizer_blank_slate' , 'on', $url );
	}
	return $url;
}

/**
 * Append the customizer url, in the Admin Menu (Appearance->Customize), with the parameters 
 * that triggers Customizer Blank Slate.
 *
 * @author soderlind
 * @version 1.0.0
 */
function add_admin_menu_customizer_blank_slate_url_param() {
	global $submenu;

	if ( isset( $submenu['themes.php'] ) ) {
		$tmp_menu = $submenu['themes.php'];
		array_walk_recursive( $tmp_menu, function( &$tmp_menu ) {
	        if ( false !== strpos( $tmp_menu, 'customize.php' ) ) {
	             $tmp_menu = add_query_arg( 'customizer_blank_slate' , 'on', $tmp_menu );
	        }
		} );
		$submenu['themes.php'] = $tmp_menu;
	}
}

functions-customize_register.php

<?php
namespace My\Customizer\Theme\Demo;

add_action( 'wp_loaded', function() {
	add_action( 'customize_register', __NAMESPACE__ . '\register_blank_slate_ui' );
	add_action( 'customize_register', __NAMESPACE__ . '\register_blank_slate_setting' );

	/**
	* Simple Customizer Demo
	*
	* TODO: Replace with actual settings
	* @param \WP_Customize_Manager $wp_customize
	*/
	function register_blank_slate_setting( \WP_Customize_Manager $wp_customize ) {
		$wp_customize->add_setting( 'etfrtb_site_name', [
			'sanitize_callback' => 'sanitize_text_field',
			'transport'         => 'postMessage',
			'capability'        => 'manage_options',
			'autoload'          => false,
		] );
	}
	/**
	* TODO: Replace with actual section and control.
	*
	* @param \WP_Customize_Manager $wp_customize
	*/
	function register_blank_slate_ui( \WP_Customize_Manager $wp_customize ) {
		register_blank_slate_setting( $wp_customize );
		$wp_customize->add_section( 'etfrtb-email-designer', [
			'title' => 'Test Section',
		] );
		$wp_customize->add_control( 'etfrtb_site_name', [
			'label'   => 'Test Label',
			'section' => 'etfrtb-email-designer',
		] );
	}
}, 9 );