This section of your admin console allows you to choose a customizable template or to create a new one. It is divided into two areas:
The menu contains three dropdown menus: choose the template that fits your needs according to the kind of message you want to send. You can choose the type of message (newsletter, Rss, or retrieve a customized template), the color and the number of sections.
Of course, once you select a template, you will be able to fully edit it in the message editor.
Create a new template using the editor. The editor resembles that of a common word processor; the buttonwill help you get the meaning of every button.
Insert the title, which is only shown to the system administrator, and set advanced options below the editor: categorize the template for an easy retrieval from the "List" menu, insert Body tag and Header.
If you are using a template for RSS feeds, you can use the RSS tags mentioned here.
You can also find a number of responsive templates, which are all fully editable.
From the MailUp blog
For more information on responsive templates, check out this blog post.