The newsletter functionality of Koala Framework is implemented using components.
You can edit your newsletter using paragraphs in the backend, where by default you have the text-image component available.
Of course you can create custom components so you can build your newsletter with exactly the functionality and design you need.
Because the newsletter component uses "Kwc_Mail_Component" to send the mail, styling the newsletter works exactly the same.
For a short overview of the newsletter visit the about page.
For a working demo try kwf-newsletter-demo.