These are the slides I used for the MagentoWA talk about Sass, Compass, Bundler and their integration with Magento themes to manage styling for a large site.

Summary

Magento Themes and Sass/Compass is a powerful combination. You can use Compass's add_import_path directive to include partials from your default theme, and use the theme asset inheritance to include your new style.css without writing any layout XML.

Diagram of Magento themes and SASS Compass styling

Modern tools for Magento development Part 2

theme sass compass magento