premailer-rails is a drop in solution for styling HTML emails with CSS without
having to do the hard work yourself.

Styling emails is not just a matter of linking to a stylesheet. Most clients,
especially web clients, ignore linked stylesheets or <style> tags in the HTML.
The workaround is to write all the CSS rules in the style attribute of each tag
inside your email. This is a rather tedious and hard to maintain approach.

Premailer to the rescue! The great premailer gem applies all CSS rules to each
matching HTML element by adding them to the style attribute. This allows you to
keep HTML and CSS in separate files, just as you're used to from web
development, thus keeping your sanity.

This gem is an adapter for premailer to work with actionmailer out of the box.
Actionmailer is the email framework used in Rails, which also works outside of
Rails. Although premailer-rails has certain Rails specific features, it also
works in the absence of Rails making it compatible with other frameworks such as
sinatra.