Not sure what to say about the error without knowing what it is in its entirety, but for the CSS, try the suggestion posted in the other thread
here and there are the the gaz_html_header and/or gaz_html_navbar and/or gaz_html_footer templates that you can edit for each style as you prefer. Use absolute http:// links, not relative links, do not use JavaScript, use inline CSS, not external CSS files, etcetera, so that you have a better chance that the receiver's email reader does not strip out parts of the HTML email.