inlinestyler is an easy way to locally inline CSS into an HTML email message.

Styling HTML email is a black art. CSS works, but only when it's been placed
inline on the individual elements (and event then, not always) - which makes
development frustrating, and iteration slow.

The general solution is to use an inlining service, which takes a message with
the CSS placed externally, and rewrites it so that all CSS is applied to the
individual elements. The most widely used of these services - and as far as I
can tell, the one that powers CampaignMonitor - is Premailer. It's a great
service, and the guys behind it put a lot of work into keeping it up to date
with the most recent discoveries in what works and what doesn't.

inlinestyler takes (most) of the functionality of Premailer, and makes it
available locally, accessible without having call a remote service.