Extensibility is one of the most important features of any software. Basically there are two types of packages to extend the functionality of Automad: themes and extensions. While Automad's template language allows beginners to develop their own themes, it is also possible to use plain PHP to develop extensions of any complexity.
It is recommended to disable caching during development!
To get started right away with developing your own package, please check out the tutorials bundled with Automad. The Tutorial package is a good and simple example for how things are organized and what are the essential parts of an Automad package.
Note that there are plugins available for some common text editors to help you developing themes for Automad.