To use child themes on Drupal 8, follow the steps below:
1. Create a new subdirectory and name it “YOURTHEMENAME-child” in the “/themes” directory.
1. Create a new CSS file with the name “YOURTHEMENAME-child.css” in the subdirectory.
1. Create a new “YOURTHEMENAME-child.info.yml” file in the subdirectory with the following content:
```
name: ‘YOURTHEMENAME Child‘
type: theme
description: ‘Child theme for YOURTHEMENAME‘
core: 8.x
base theme: ‘YOURTHEMENAME‘
```
1. In the Drupal 8 administration panel, go to “Appearance” and select your new child theme.
1. Enable the child theme by clicking the “Enable and set default” button.
1. Customize the child theme by modifying the CSS file created in step 2.
1. Save your changes and test the new child theme.
