The major advantage of using enews extended is that it matches the StudioPress theming for the existing enews widget. In other words, if you're trying to make a site look like the demo and really like how they have the subscribe form, enews extended will give it to you without editing style.css
As far as how to edit the style of it, it is the same as the enews widget. In style.css, search for "enews" and that'll cover you (assuming the theme has something already in place for the stock enews widget).
(note: I'm the dev behind eNews Extended)