Deliverance can be used to theme a site without having to have any knowledge of Plone.
After installing Deliverance, you can use paster to create a Deliverance project. Deliverance uses “rules” to fetch HTML from a source and place it in the theme you create. The source site doesn’t have to be a Plone site by the way. It can also be e.g. a PHP site or even a static site. As long as there is HTML to process.
Deliverance is taking the resources (like the CSS files) from the source and the theme itself and combines them. This makes the Kupu editor work for example, but it can also result in strange situations where the styles clash. The HTTP headers of the main content, like the last modified header, are (almost) forwarded unchanged. So caching with Squid is still possible.
Deliverance is, however, limited to modifying the HTML it received. It doesn’t know anything about the content. Reordering is possible, hiding certain elements is also possible. Still, you regularly have to dive into the underlying framework (Plone) to change the end result.
See the Deliverance documentation for more information.