Override editor colors in (twenty eleven) child theme

custom-editor-style.css

/*
Theme Name: Twenty Eleven
Description: Custom colors.
*/

body {
  color: #fff;
  background: #ddd;
}

functions.php

<?php

/**
 * Registers an editor stylesheet for the theme.
 */
function wpdocs_theme_add_editor_styles() {
	add_editor_style( 'custom-editor-style.css' );
}
add_action( 'after_setup_theme', 'wpdocs_theme_add_editor_styles' );