The theming in Drupal 8 has been changed. This is primarily due to the transition to object-oriented programming and the use of the twig template engine. If earlier it was enough to learn a little PHP, HTML, CSS and start creating your own theme, now you need to figure out twig as well. On the one hand, this complicates the development of Drupal, but on the other hand simplifies the development process. Using a template engine is more convenient for a group of developers when one specialists specializes in developing a function, while others specialize in developing a frontend site.
It is necessary to gradually master each new technology and development approach: SASS, Twig, Grunt, bootstrap - all this has long become the standards for developing web applications. So let’s and we will figure out how to use all this.