You can’t really create a “grandchild” theme in the normal sense – i.e make it the child theme of a child theme. It might technically be possible (Smashing Magazine said it was back in 2013) but it seems to be generally considered to be not “WordPress legit”.
What you can do is create the grandchild theme as a plugin – although be aware that it tends to be considered as bad practice because it’s an indication of bad architecture, and if you need a grandchild theme “something is wrong”. However I can see the logic if you are using a third party theme that is itself a child theme.
Should you use a grandchild theme?
The main practical difficulties with grandchild themes are they they adds another level of “moving parts” into your site, making it more difficult to maintain and debug.
The references I’ve listed below discuss the practicalities and disadvantages of grandchild themes, so you can decide if this is still an option for you:
- WP Smith: A Theme Framework, Child Themes, & Grandchild Themes
- Justin Tadlock:
- Tom J. Nowell’s answer on wordpress.stackexchange.com to the question “Is it possible to make grandchild themes?”
How to create a “grandchild theme” using a plugin:
If you decide that this is the way to go, the following describe how to go about creating a plugin for your “grandchild theme”
Alternatives to grandchild themes:
Another suggestion is to make a copy of the child theme to customise, although this means it’s no longer easy to upgrade the child theme.
This article offers a way of editing the child theme in a way that “minimizes the coupling between the changes you make and the original child theme” …meaning you don’t need a grandchild theme and the changes to your child theme should (hopefully) be minimal, making it easier to upgrade.