Echo wraps content in a themed webpage. Echo can be added to HTML Mail to produce mail with your Web site theme.
See http://drupal.org/project/echo for details and download.
Echo themes the content by calling URL /echo and retrieving the themed result. Echo uses the theme passed to Echo by the calling module.
I am using the HTML mail module. HTML mail lets you select a theme for the mail then passes the theme machine name to Echo. You could set up a subtheme for your mail then specify the subtheme in HTML mail. HTML mail will switch off theming for users who select the plain mail option in their profile.
I do not see a way to change the theme by content type. Would be nice if Mail edit let you select a theme then pass it through HTML mail to Echo.
Here is the README file:
Wraps content in a themed webpage.
This very simple module contains only one externally useful function:
echo_themed_page($title, $content, $theme)
Returns a page themed for the anonymous user.
Generates the text of a fully-themed Drupal page. The rendered
page is returned as a text string instead of being sent to the
browser. The theme system can thus be used to style any HTML
message as if it had been generated by the live website.
$title The text to display as the page title.
$content The text to display in the main page content
$theme The machine-readable name of the theme to use.
A string containing the fully-themed page.