Technical requirements
Given this is an advanced chapter, some specific technical knowledge is assumed. Themes in Drupal leverage CSS, JavaScript, and HTML. This book does not cover these topics in depth given they are not specific to Drupal.
As stated earlier, functionality built in Drupal is typically put together using entities, views, display modes etc. All of these exist outside of the theme layer. The theme layer exists to change markup around existing functionality, and add the ability to style and add interactivity to it.
As of the time of this writing, Drupal’s paid theme ecosystem is very small (compared to other CMSs such as WordPress) because of several reasons:
It’s hard to style functionality that the theme author doesn’t know will exist
Most Drupal sites tend to be on the larger side, and require custom bespoke designs
