The most obvious part of Drupal's theming system is the Appearance admin page found at admin/appearance, which lists all the themes installed on your website. The page is shown in the following screenshot:
When you choose a theme from the Appearance page, you are applying a specific graphic design to your website's data and functionality. However, the applied theme is in reality only a small part of the entire theming layer.
This book mostly focuses on building modules that encapsulate chunks of a functionality. However, since we're ultimately building a web application, everything outputted by our functionality will need to be marked ...
