Changing editor-style.css style

It doesn’t work because WordPress isn’t loaded within the context of editor-style.php, and you’re actually getting a fatal error (get_theme_mod undefined). Enable error logging and you’ll see what I mean.

Rather than pointing directly to a custom PHP file, you should use an “endpoint” within WordPress:

add_editor_style( admin_url( 'admin-ajax.php?action=editor-style' ) );

Now you can “listen” for when this virtual stylesheet is requested and kick out all your awesome CSS with WordPress loaded:

function wpse_226550_editor_style() {
    header( 'Content-Type: text/css; charset=UTF-8' );
    echo $css; // Example!
    exit;
}

add_action( 'wp_ajax_nopriv_editor-style', 'wpse_226550_editor_style' );
add_action( 'wp_ajax_editor-style', 'wpse_226550_editor_style' );

Don’t be put off by the “ajax” reference – admin-ajax.php is a popular technique in WordPress for sending all kinds of custom responses, but without having to manually load WordPress yourself (e.g. require './path/to/wordpress/wp-load.php'; – bad practice, path assumptions).

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