A theme is a collection of files that define the presentation layer. You can also create one or more "sub-themes" or variations on a theme. Only the .info file is required, but most themes and sub-themes will use other files as well. The following diagram illustrates the files that are found in a typical theme and sub-theme. Drupal 7 .info (required) All that is required for Drupal to see your theme is a ".info" file. Should the theme require them, meta data, style sheets, JavaScripts, block regions and more can be defined here. Everything else is optional.