HTML-MAIL: How to setup the templates and how to apply the templates to setup a newsbrief

Dear everyone,

For the volunteer organization which I join as volunteer I am trying to get a
newsletter up and running, but somehow, I can't get the template/layout
integration correct. Could anyone help me out, how I can build it and install it correct.

Sending HTML e-mail is working, only not the layout/theme.

Current situation:

Drupal 7 (latest stable release)
Mailsystem
Mimemail
HTMLmail
Simplemail news module
Current site theme: Adaptive Theme with subtheme Sky.

Currently the idea is to set up a newsletter like this example for a
colleque site owner:

http://www.vlaamspaardenloket.be/nieuwsbriefnieuw/index.php?id=9

Could you inform me how to set up the correct code in the template (PHP) and how apply the template

Yours Sincerely,
Johan Heegsma