error 403

Hi,

This means that you have a security system which blocks the saving.
You have to see if you can exclude the email saving area of the backend from it.

Or you'll have to customize the emails by changing the files in the media/com_hikashop/mail/ folder via FTP.
In there, you can make copies of the files, adding .modified at the end to create overrides.
So the file order_notification.html.php would be copied to order_notification.html.modified.php
That way, you won't loose your modifications in the override files.

Hi,

Not all CSS code can be used in emails.
Some email clients block some CSS. Gmail blocks quite a lot of CSS.
mailchimp.com/help/limitations-of-html-email/