Sidebar Layout Option im WordPress Customizer

customizer.php

<?php
/**
 * Adds Layout Options section and sidebar position setting.
 *
 * @param WP_Customize_Manager $wp_customize The Customizer object.
 */
function theme_slug_customize_register( $wp_customize ) {

	// Add layout options section.
	$wp_customize->add_section( 'theme_slug_layout_options', array(
		'title'    => esc_html__( 'Layout Options', 'theme-slug' ),
		'priority' => 120,
	) );

	// Add featured category setting and control.
	$wp_customize->add_setting( 'sidebar_position', array(
		'default'           => 'right',
		'sanitize_callback' => 'theme_slug_sanitize_select',
		'transport'         => 'refresh',
	) );

	$wp_customize->add_control( 'sidebar_position', array(
		'label'    => esc_html__( 'Sidebar Position', 'theme-slug' ),
		'section'  => 'theme_slug_layout_options',
		'type'     => 'select',
		'priority' => 1,
		'choices'  => array(
			'left'   => esc_html__( 'Left Sidebar', 'theme-slug' ),
			'right'  => esc_html__( 'Right Sidebar', 'theme-slug' ),
		),
	) );

}
add_action( 'customize_register', 'theme_slug_customize_register' );

extras.php

<?php
/**
 * Adds custom classes to the array of body classes.
 *
 * @param array $classes Classes for the body element.
 * @return array
 */
function theme_slug_body_classes( $classes ) {

	// Switch sidebar to left.
	if ( 'left' === get_theme_mod( 'sidebar_position', 'right' ) ) {
		$classes[] = 'left-sidebar';
	}

	return $classes;
}
add_filter( 'body_class', 'theme_slug_body_classes' );

style.css

@media screen and (min-width: 56.875em) {
  .left-sidebar .sidebar {
      float: left;
      margin-right: 75%;
      margin-left: 0;
  }

  .left-sidebar .content-area {
      float: right;
      margin-left: -100%;
      margin-right: 0;
  }
}