A base theme is someone's idea of a really great starting point. It might include the stub files for CSS overrides, template files which bring Drupal core's markup up-to-date with HTML5, useful theme overrides which create new template variables. In other words: a base theme can provide you with a lot of really great stuff! The first version of the Domicile Theme used the NineSixty base theme—just enough for a simple site, but not so much overhead that it was complicated to learn. This version of the theme doesn't use a base theme. That's because, thanks to Sass, we'll be able to get almost all of the power of the NineSixty base theme through a gem plug-in. In this lesson you will learn which folders are typically used to keep Drupal themes neat and tidy.
Check your version
This video covers a topic in Drupal 7 which may or may not be the version you're using. We're keeping this tutorial online as a courtesy to users of Drupal 7, but we consider it archived.