using new messages css file filters

Messages_CSS_File_Filter_Example.php

<?php
function ee_inline_css_swap( $path_or_url, $type ) {
	$upload_dir = wp_upload_dir();
	if ( $type == 'preview' ) {
		$path_or_url = $upload_dir['baseurl'] . '/email-messenger-inline-preview-css.template.css';
	} elseif ( $type === FALSE ) {
		$path_or_url = $upload_dir['basedir'] . '/email-messenger-inline-css.template.css';
	}

	return $path_or_url; 
}
add_filter( 'FHEE__EE_Email_Messenger__get_inline_css_template__css_url', 'ee_inline_css_swap', 10, 2 );
add_filter( 'FHEE__EE_Email_Messenger__get_inline_css_template__css_path', 'ee_inline_css_swap', 10, 2 );