There’s a new built-in theme available on Micro.blog called Lanyon, based on the open source Hugo theme of the same name. It’s a clean, readable design with a slide-out menu for page links.
I’ve been working on getting several new themes ready for Micro.blog. Some Hugo themes work with only a couple tweaks, and some take more time. My fork of Lanyon with Micro.blog-specific changes is on GitHub.