The responsibility for theming a given piece of data is spread out over the Drupal core, modules, and the themes themselves. So, as a module developer, it is important to know that both modules and themes can theme data or content.

In this book, we will focus on the aspects that happen at the module level. We will not concern ourselves with styling, but work primarily with theming definitions and templates that are needed within the module. Typically, it is best practice to ensure that modules are able to theme their data. If done right, themes can then come into play to style the output or override that theming to change the presentation altogether.

A major shift in Drupal 8 compared to older versions is the move to the open source Twig templating system (https://twig.sensiolabs.org/). This makes the separation of logic from a presentation that much clearer and makes frontend developers jobs much easier, not to mention more secure.