Themes

A theme is a reusable set of layouts and assets. Put one under themes/<name>/ with its own layouts/, static/, and assets/, and select it in blogin.json:

"theme": "acme"

The theme's files fill in behind your own, file by file. A layouts/show.haml in your site overrides the theme's show.haml while the theme still supplies base.haml, index.haml, and the rest. Your static/ and assets/ win over the theme's on the same path, so you can replace a single stylesheet without forking the theme.

Per-section layout overrides resolve through the theme too, so a theme can ship layouts/essays/show.haml and your site can override just that one file.

A theme is not a css-framework. A framework maps element classes and nothing else, while a theme brings whole templates and assets. The two combine, and CSS Frameworks covers the framework side.