Clarity is an attempt to get back into Hubzilla theme development. At present, it is the only theme in my repo that will work with the latest versions of Hubzilla. Everything else needs to be updated to work with the latest interface improvements.
The design intent around Clarity is to configure a good basic layout, and polish up Hubzilla's look and feel. Work is ongoing to give the theme some sense of uniqueness.
It's not too hard to hack a theme together; mostly you just need to create some PHP files denoting what the theme name is, and what the CSS component should be called.
Schemas are preferable if you anticipate having variations of the same theme. Personally, I disable these and try to create single-style themes instead.
The other cool thing is that Hubzilla themes support custom widget layouts. I believe they might also support entirely original widgets written from scratch, but I haven't explored that a whole lot yet.