How to create _extend_child.less for grand children theme

Question

I am new to Magento 2. I am having difficulty in creating a child theme. However I have created the child theme now I am facing how to add source for child them.

In my configuration, I have created child theme of Luma, which is child theme of Magento blank theme. I want to extend all css from either parent or grand parent. So decided to not give same name which already exist in parent theme (luma).

I am planning to add _extend-child.less to the parent theme and keep it blank. And creating same filename in child theme where I can add my own CSS or can add call child sources. Now my question is:

1. As luma theme is primarily present in vendor directory. I cannot add _extend-child.less here as it might override on upgrade. How could I copy this to app/design/frontend/Magento directory?
2. Does copying the complete folder vendor/magento/theme-frontend-luma/ to app/design/frontend/Magento/ would work?
3. Does changing code on app/design/frontend/Magento/ would not be affected on theme update/upgrade?
4. Any standard way to include my child CSS to luma theme.

Thanks in advance.

Answer 1

If you extend from Luma, you have all CSS of Luma. If you want to extend e.g. Magento_Checkout, you would create _extend.less in Magento_Checkout/web/css/source

If you want to override this CSS completely you would create _module.less.

https://devdocs.magento.com/guides/v2.4/frontend-dev-guide/css-guide/css_quick_guide_approach.html