Any alternate TinyMCE4 themes / subthemes?

TinyMCE4 in WordPress 3.9 has some cool changes, but it seems to have only one “theme” included.

Is there any way to make a sub-theme of the theme defined by CSS, and theme.js in wp-includes/js/tinymce/themes/modern ? Or a way to dequeue and enqueue a custom theme from somewhere else? Where can I find these MCE4 themes, if it’s a common pluggable option?

I’m also really curious what the tinymce.ThemeManager.get('modern') is meant for, since it only gives you the function(editor) {... } containing private variables and functions from theme.js, but gives you no way to “monkey patch” the reposition or add-toolbar functions, etc. that are within that function.

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