Legacy code to enqueue a theme or child theme's version of frontend.css for PMPro v3.1+.

my_pmpro_enqueue_theme_frontend_css.php

<?php
/**
 * Enqueue your theme or child theme's version of the legacy frontend.css file.
 * Note: PMPro v3.1+ css selectors have been overhauled and many of the legacy class names are no longer supported. 
 */
function my_pmpro_enqueue_theme_frontend_css() {
	if ( is_admin() ) {
		return;
	}

	// Figure out which frontend.css file to load.    
    if( file_exists( get_stylesheet_directory() . "/paid-memberships-pro/css/frontend.css" ) ) {
        $frontend_css = get_stylesheet_directory_uri() . "/paid-memberships-pro/css/frontend.css";        
    } elseif( file_exists( get_template_directory() . "/paid-memberships-pro/frontend.css" ) ) {
        $frontend_css = get_template_directory_uri() . "/paid-memberships-pro/frontend.css";        
    }

	if ( empty( $frontend_css ) ) {
		return;
	}

	wp_enqueue_style( 'my_pmpro_custom_frontend', $frontend_css, array(), '1.0', 'all' );

}
add_action( 'wp_enqueue_scripts', 'my_pmpro_enqueue_theme_frontend_css', 1 );