Wordpress Custom TinMCE style.

functions.php

add_filter('tiny_mce_before_init', 'tinymce_before_init_extend');
add_filter('teeny_mce_before_init', 'tinymce_before_init_extend');
function tinymce_before_init_extend($atts)
{
    $custom_options = array(
        'content_css' => get_bloginfo('template_url') . '/css/editor.css'
    );

    return array_merge($atts, $custom_options);
}