add a class to ALL widgets?

No, the plugin will only save the class for individual widgets. If you need a class for all widgets I would modify your theme’s register_sidebar call (often in functions.php) or even easier, just use a CSS selector that targets all your widgets ( #sidebar aside.widget for example).

register_sidebar( array(
			'name'          => __( 'Header Widget Area', 'theme' ),
			'id'            => 'header-widget-area',
			'before_widget' => '<aside id="%1$s" class="widget %2$s">',
			'after_widget'  => '</aside>',
			'before_title'  => '<h1 class="widget-title">',
			'after_title'   => '</h1>',
		) );

I have about 20 sidebars… too much effort.

I ended up just using jquery to addClass.

Thanks heaps for the quick reply!!