This module automatically converts an external set of stylesheets to inline CSS for transactional email templates, to make it easier to write and maintain them. It also comes with some responsive templates that you can use and customize.
Install using modman.
modman clone responsive-email git@github.com:magento-hackathon/Responsive-Email-Templates-with-CSS-Inliner.git
Customize your New Account email in System > Transactional Emails, using the HTML in template/email_responsive/ink/account_new.html
Also, put /** external-responsive-css */ in the Template Styles field.
Note: An alternate template is available at template/email_responsive/basic/account_new.html - if you use it, change Mail Style Settings to use basic.css for inline and basic-media-query.css for non-inline.
Then test it out using preview or by creating an account to generate a welcome email to yourself.
If you'd like to use SASS, just drop a file in skin/frontend/base/default/sass/responsive_email,
and from skin/frontend/base/default/ run compass watch. The css will be generated into
skin/frontend/base/default/css/responsive_email. See basic.scss as an example.
You can add your own stylesheet to add some customization on top of our base CSS by going into System > Configuration > System > Mail Style Settings.