Designers have the ability to import HTML that includes MailChimp's template language to create an editable template inside our app. This is a great feature for web designers who want to upload a template for a client, and create editable, repeatable and hideable sections within the content area (without letting clients uglify the rest of your design).
If you're not experienced with HTML and would like to create a custom template, you can still customize one of our templates to use for your campaigns.
Build Your Template
- To get started select the Templates option from the MailChimp Dashboard. On the templates page click Select under the "Code Your Own" option to pull up the Custom Template Builder screen. Or you can select "Import" if you've already got your HTML ready.
- In the code panel in the bottom half of the screen, you'll notice a little markup where you can change the "tab" (@tab) to be called, for example, "Booya" instead of the default "Page" (you can name the tabs anything you want), and you can also change the "tip text" (@tip) to your liking.

You can also do a bunch of other cool stuff, like change all the default fonts, background colors — basically anything you want. If you're designing an e-commerce style template with a product matrix or grid, you can set it up with placeholders for graphics and text and then make the entire section repeatable:
When you (or your client) uses this template, they can just fill in the blanks by adding pics and text. They can click the green (+) icon to add another row.
Simple Markup
The cool thing about all this is, you can modify the MailChimp template design interface by simply adding a few comments to your HTML source. Add tabs, modify tabs, or remove tabs altogether.
Once you're done you can save the template as Newsletter Template (or whatever name you want to use) and it'll appear as an option for future campaigns:
So, if you're a designer building out some templates for one of your clients, you could create several options for them and name them newsletter, holiday_sale, birthday_autoresponder, and so on.
After you select New Welcome Email, it takes you to the campaign builder. And as you can see, the first tab at the top is "Booya" and the tip text is new:
More how to on the tags used to create the custom templates can be found in the documentation here