This is just a sketch, primarily for personal use, but anyone is welcome.

Combining modular drupal.css with forms api can potentially revolutionise theme packs with drupal.

Just consider this. If you have colour, framing, typography and icons split into different stylesheets, all you need to do to instruct the theme to use an alternative is to use a different color.css for example. This will introduce a different colour scheme, preserving everything else. $element['head']['style'][]='colour-1.css' kind of thing.

And this is just the tip of the iceberg. What can prevent you from rendering the content into a non-html format using the themes only - RSS 1.0, RDF, OWL, RSS 2.0, Atom - essentially all of them share a lot with a node listing, only the syntax and the feature subset differ.