WordPress Widget Order via CSS; Here is a simple filter to automatically add a class attribute like widget-order-1 to all widgets within sidebars

example.php

add_action( 'init', 'add_widget_order_class' );
function add_widget_order_class() {
	global $wp_registered_sidebars, $wp_registered_widgets;

	$sidebars = wp_get_sidebars_widgets();

	if ( empty( $sidebars ) )
		return;

	foreach ( $sidebars as $sidebar_id => $widgets ) {

		if ( empty( $widgets ) )
			continue;

		foreach ( $widgets as $i => $widget_id ) {
			$order = $i + 1;
			$wp_registered_widgets[$widget_id]['classname'] .= ' widget-order-' . $order;
		}

	}

}