WordPress Classic Editor Styles

wds_classic_editor_styles.php

<?php
function my_plugin_add_editor_style() {
    $editor_style_url = plugins_url( 'css/editor-style.css', __FILE__ );
    $editor_style_path = plugin_dir_path( __FILE__ ) . 'css/editor-style.css';
    $editor_style_version = filemtime( $editor_style_path );
    add_editor_style( $editor_style_url . '?ver=' . $editor_style_version );
}
add_action( 'admin_init', 'my_plugin_add_editor_style' );