add_editor_style()

function add_editor_style( $stylesheet = 'editor-style.css' ) {
	global $editor_styles;

	add_theme_support( 'editor-style' );

	$editor_styles = (array) $editor_styles;
	$stylesheet    = (array) $stylesheet;

	if ( is_rtl() ) {
		$rtl_stylesheet = str_replace( '.css', '-rtl.css', $stylesheet[0] );
		$stylesheet[]   = $rtl_stylesheet;
	}

	$editor_styles = array_merge( $editor_styles, $stylesheet );
}

/**
 * Registers an editor stylesheet for the current theme.
 *
 * @global GC_Post $post Global post object.
 */
function gcdocs_theme_add_editor_styles() {
	global $post;

	$my_post_type = 'posttype';

	// New post (init hook).
	if ( false !== stristr( $_SERVER['REQUEST_URI'], 'post-new.php' )
			&& ( isset( $_GET['post_type'] ) === true && $my_post_type == $_GET['post_type'] )
	) {
		add_editor_style( get_stylesheet_directory_uri() . '/css/editor-style-' . $my_post_type . '.css' );
	}

	// Edit post (pre_get_posts hook).
	if ( stristr( $_SERVER['REQUEST_URI'], 'post.php' ) !== false
			&& is_object( $post )
			&& $my_post_type == get_post_type( $post->ID )
	) {
		add_editor_style( get_stylesheet_directory_uri() . '/css/editor-style-' . $my_post_type . '.css' );
	}
}
add_action( 'init',          'gcdocs_theme_add_editor_styles' );
add_action( 'pre_get_posts', 'gcdocs_theme_add_editor_styles' );

function value( $ar, $key, $default = '' ) {
	if ( is_array( $ar ) && isset( $ar[ $key ] ) ) { return $ar[ $key ]; }
	//else

	return $default;
}

/**
 * Registers an editor stylesheet for the current theme.
 *
 * @global GC_Post $post Global post object.
 */
function gcdocs_theme_add_editor_styles() {
	global $pagenow;
    global $post;
    $my_post_type = 'posttype';
 
    if ( ( ( 'post-new.php' === $pagenow ) && ( value( $_GET, 'post_type' ) == $my_post_type ) ) ||   // New post (init hook)
         ( ( 'post.php' === $pagenow ) && is_object( $post ) && ( get_post_type( $post->ID ) == $my_post_type ) ) ) {  // Edit post (pre_get_posts hook).
        add_editor_style( get_stylesheet_directory_uri() . '/css/editor-style-' . $my_post_type . '.css' );
    }
}

add_action( 'admin_init',    'gcdocs_theme_add_editor_styles' );
add_action( 'pre_get_posts', 'gcdocs_theme_add_editor_styles' );