woocommerce_email_styles │ filter-hook │ WC 1.0

Usage

add_filter( 'woocommerce_email_styles', 'wp_kama_woocommerce_email_styles_filter', 10, 2 );

/**
 * Function for `woocommerce_email_styles` filter-hook.
 * 
 * @param  $ob_get_clean 
 * @param  $that         
 *
 * @return 
 */
function wp_kama_woocommerce_email_styles_filter( $ob_get_clean, $that ){

	// filter...
	return $ob_get_clean;
}

$ob_get_clean

-

$that

-

Where the hook is called

$css = apply_filters( 'woocommerce_email_styles', ob_get_clean(), $this );

Where the hook is used in WooCommerce