How to add css to the TinyMCE editor

Does anyone know how to add CSS styles to the TinyMCE

I have a file in a mu-plugin where I can add styles with this code:

add_action('admin_head', 'cm_styles');
add_action('login_head', 'cm_styles');
add_action('customize_controls_print_scripts', 'cm_styles');
function cm_styles() {
   echo '<style type="text/css">
/* custom styles */
	</style>';
}

I can style anything in the admin/login/ pages and the top toolbar. I have tried to add every type of hook I could find relating to the mce editor to add to the above code such as:

add_filter( 'mce_css', 'cm_styles' );

but I can’t get it to work.

Any help appreciated