The readme.md file states: "Latest versions of the theme will not override custom-style.css file."
If composer is used (highly recommended by Drupal), this css file is deleted. Composer installs a new version in a temporary directory. After correct installation, the old directory is deleted and the temporary directory is given the correct name. So there is no way to save the css file.
Please correct the incorrect documentation.
Active
1.24
Documentation