Transacction PHPList Emails make them HTML instead Plain Text

There are few emails for transaction messages such as “Confirmation of subscription”, “Welcome” and “Goodbye” we have modified them in HTML but they are being send in Plain Text, is there any variable or option I should change to make it in HTML?
Thanks for the feedback.

Already figured out by myselft how to perform the changes I wanted.

  1. You have to setup a template then assign it as the system default
  2. You have to modify the transaction messages, and now then you can insert some html tags for style

That’s it. I just did a wonderful work in my system notifications.

Thanks @michield for you help!