Look for a stylesheet in a WP child theme or theme before loading a default plugin stylesheet.

override_plugin_stylesheet.php

//figure out which css to load
if(file_exists(get_stylesheet_directory() . "/pluginname/frontend.css"))
	$css_url = get_stylesheet_directory_uri() . "/pluginname/frontend.css";
elseif(file_exists(get_template_directory() . "/pluginname/frontend.css"))
	$css_url = get_template_directory_uri() . "/pluginname/frontend.css";
else
	$css_url = plugins_url('css/frontend.css',dirname(__FILE__) );

//load it
wp_enqueue_style('pluginname-frontend', $css_url);