Any alternate TinyMCE4 themes / subthemes?

I’m not sure if this question is still relevant or not (so I’m posting for future reference for people), but there is a way to define a css file for the TinyMCE editor in your plugin by using the mce_css filter. For a quick example, I’ll take an excerpt from the WordPress documentation:

function plugin_mce_css( $mce_css ) {
    if ( ! empty( $mce_css ) )
        $mce_css .= ',';

    $mce_css .= plugins_url( 'editor.css', __FILE__ );

    return $mce_css;
}
add_filter( 'mce_css', 'plugin_mce_css' );

For reference on this filter, you can check out these pages:

mce_css @ wpseek.com

mce_css @ codex.wordpress.org