This is a plug-in for Micro.one and Micro.blog that adds a settings interface for editing the custom footer. The footer in this case isn't necessarily part of the design, although it can be. This is more commonly used to add a snippet of JavaScript or HTML that you want to be inserted into your blog at the very end of every page, right before the closing </body>
tag.
For complete control over the design of footers and navigation on your blog, you can install new themes or edit an existing template in Micro.blog. (Editing themes isn't available with a Micro.one subscription.)