Remove Drupal core stylesheets with hook_css_alter

remove_core_css

<?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]);
        }
    }
}