Add Logic with THEMENAME.theme for Drupal 8, 9, and 10

Every theme can contain an optional THEMENAME.theme file. This file contains additional business logic written in PHP and is primarily used for manipulation of the variables available for a template file, and suggesting alternative candidate template file names. Themes can also use this file to implement some, but not all, of the hooks invoked by Drupal modules.

In this tutorial we'll learn:

  • The use case for THEMENAME.theme files, and where to find them
  • The different types of functions and hooks you can implement in a THEMENAME.theme file

By the end of this tutorial you should be able to know how to start adding PHP logic to your custom theme.