WordPress default theme is based on Kubrick, developed by M. Heilemann. Kubrick is now part of WordPress, and Heilemann is working on K2, a more advanced theme framework.
I installed v0.9.5 RC1 in about 5 min, with no problems at all. Configuration of the sidebar took for another 10 min tops. My first impression is that it’s definitely an improvement over Kubrick, and I really like it.
I also installed the Extended Live Archives plugin, and enabled the Archives tag in the blog. K2′s archive system is quite impressive.