Mandaris Moore has written the first of a series documenting how he created a new theme for Micro.Blog — Creating Labarum (Part 1): Design and initialization:

I wanted to create a new theme for, and I thought I would document all the steps that I went through. The creation of the theme was an iterative process for me and this tutorial works in that way.

This is comprehensive and informative. Look out for the continuation of the series.