Flagged by Jose Jimenez:
Article Link: https://help.liferay.com/hc/en-us/articles/360028726552
Comment: "We need to notify that the option of extending a theme installed in the local registry (globally installed theme) can make the project not be portable to other machines, for example if you have a Continuous Integration tool you should extend themes from a URL or published in the npm registry. Anyway, if you are extending a custom theme in the same project, you could use the 3) option, and modify the paths to the module in the package.json and package-lock.json, and set relative paths to the location where you have your other base custom theme. Usually it will be similar to "../my-base-custom-theme" because in a Liferay Workspace all the themes are in the same "themes" folder. This is a workaround portable to other machines, in a future we will try to improve the way we do this task. (https://github.com/liferay/liferay-js-themes-toolkit/issues/522)
NOTE FOR THE REVIEWER: this could be part of a Fast Track but I think is better to put the workaround in place with this documentation in order to avoid that people following this documentation gets the error "