1. In the root directory of your Drupal site, create a folder called mytheme in the themes folder.
2. Inside the mytheme folder, create a mytheme.info.yml file so that Drupal can discover the theme. We will then edit this file:

3. First, we will need to define the themes name using the name key:

name: My Theme 

4. All the themes need to provide a description key, that will be displayed on the Appearance page:

description: My custom theme 

5. Next, we will need to define the type of extension, that is, a theme and the version of core that is supported:

type: theme 
core: 8.x 

6. The base theme call allows us to instruct Drupal to use a specific theme as a base:

base theme: classy 

7. The final item is a regions key that is used to define the regions of the blocks that can be placed, which is a YAML-based array of key/value pairs:

regions: 
  header: 'Header' 
  primary_menu: 'Primary menu' 
  page_top: 'Page top' 
  page_bottom: 'Page bottom' 
  breadcrumb: 'Breadcrumb' 
  content: 'Content' 

Regions are rendered in the page template file, which will be covered in the next recipe, Twig templates.

8. Log in to your Drupal site, and go to Appearance from the administrative toolbar.

9. Click on Install and set default in the My theme entry to enable and use the new custom theme: