A custom theme might sound scarier than it is. In this case, you would only touch a file called config.json which is for changing settings that are not exposed in the Micro.blog user interface. It would not affect the design of your blog.
The config.json file contains many lines of text, but the relevant one looks like this:
"description": "Follow <a href=\"https://micro.blog/[USERNAME]\">@[USERNAME] on Micro.blog</a>.",
That can be changed into something like this:
"description": "Follow <a href=\"https://example.com/user\">@user@example.com</a>.",
Totally understand if you don’t want to make the change, though. An alternative solution is to just hide the subheading using Custom CSS:
.blog-description {
display: none;
}