I added a new built-in design to Micro.blog today: Archie, based on the Hugo theme of the same name. It has been lightly modified to work well in Micro.blog, and my changes are on GitHub.
What I liked about this theme is that it was different than most of the built-in designs, and it has a custom dark mode interface:
The link colors can be changed using a custom theme by editing the top of the
✴️ Also on Micro.blog