Build editor stylesheet from customizer settings

editor-styles-from-customizer.php

<?php
// Place in functions.php of Twenty Sixteen to see editor background take on color of customizer background.

/**
 * Build editor stylesheet from customizer settings upon customizer save.
 */
function kwh_build_stylesheet() {
  $upload_dir = wp_upload_dir();
  $stylesheet = $upload_dir['basedir'] . '/kwh-editor-style.css';
  $styles = '';

  // Get customizer settings to be written to stylesheet.
  $background_color = get_theme_mod( 'background_color' );

  // Construct styles.
  // Note "\n" must be in double quotes in order to be escaped.
  $styles .= 'body { background-color: #' . $background_color . '; }' . "\n";

  // Write styles to file.
  file_put_contents( $stylesheet, $styles  );
}
add_action( 'customize_save_after', 'kwh_build_stylesheet' );

/**
 * Add editor stylesheet so customizer styles appear in editor.
 */
function kwh_add_editor_style() {
  $upload_dir = wp_upload_dir();
  $stylesheet_url = $upload_dir['baseurl'] . '/kwh-editor-style.css';
  add_editor_style( $stylesheet_url );
}
add_action( 'admin_init', 'kwh_add_editor_style' );