Remove Drupal's core system stylesheets

template.php

<?php
/**
 * Remove Drupal's core system stylesheets.
 */
function THEMENAME_css_alter(&$css)
{
	$path_system = drupal_get_path('module', 'system');
	
	$remove = array(
		$path_system . '/system.base.css',
		$path_system . '/system.menus.css',
		$path_system . '/system.messages.css',
		$path_system . '/system.theme.css',
	);
	
	// Remove stylesheets which match our remove array.
	foreach ($css as $stylesheet => $options) {
		if (in_array($stylesheet, $remove)) {
			unset($css[$stylesheet]);
		}
	}
}