It's just an example, you can as well transform it into one for customer area, just have a look at how it is formatted, at the entry script and you'll understand how to create one for customer area as well.
Just a hint, if you create a theme for customer, say a theme named "whatever", it must contain only:
--whatever (folder containing theme files)
----- WhateverTheme.php - the entry file, copy it from the demo theme and keep only the run() method, and empty that method
----- views - the folder that can contain folders to override the ones from customer/views folder
Then just pack it, go to Mailwizz -> Backend -> Extend -> Themes, click on the customer tab and install it from there, then simply copy folders from apps/customer/views into the theme view folder in order to override the defaults.
It's harder to explain, in practice it's very simple actually.