For what it's worth, I chose to go with IntelliContactPro instead of Constant Contact back in 2004 and I've yet to regret doing so.
I must admit I'm out of my depth when you mention CSS in creating a newsletter and then pasting it into "their software." But ICP includes a WYSIWYG page that allows you to switch to and fro with its HTML version. They also have a selection of templates, although I prefer to use my own design.
Moreover, their support is first rate and includes a Live Chat line.
Perhaps best of all as far as you're concerned, though, is a demo page that walks you through everything and then allows you to sign up for a trial run.
No, I'm not one of their affiliates (even though, like Constant Contact, they have them), but you might well find them worth a look. Their home page is at
http://www.intellicontact.com/
Duncan