I use Metalsmith. It's a very bare-bones static site generator where everything is a plugin. So you get to define your own workflow.

A workflow could be:

1. Take Markdown files from a source directory
2. Get the right template (written in pretty much any templating language)
3. Update all links
4. Generate pretty permalinks
5. Generate the whole website
6. Deploy and go live.

You could, technically, use themes, but I don't know how many you'll find out in the wild. Everyone has their own personal workflow/templates.

I've written a beginner-level Metalsmith tutorial on my blog (which itself is powered by Metalsmith), if you're interested.