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<?php 

/* Replace Standart WP Menu Classes */
function change_menu_classes($css_classes) {
        $css_classes = str_replace("current-menu-item", "active", $css_classes);
        $css_classes = str_replace("current-menu-parent", "active", $css_classes);
        return $css_classes;
}
add_filter('nav_menu_css_class', 'change_menu_classes');

?>