Is it possible to have a child theme of a child theme?

Hello together,
I have a highly customized mainroad theme for my website. But addionally some of my websites need incompatible customizations to this “customized base theme”. Currently I’m handling it the following way:
Base theme in “themes/mainroad” and customizations in layouts/.
So it’s still possible to implement updates of the base theme. But to handle extra customizations for some sites I would prefer something like:
themes/my_base_customized_theme/themes/mainroad ← The base mainroad theme
themes/my_base_customized_theme ← My customized theme which inherited of the mainroad theme
layouts/ ← special customizations for specific websites.

This would make it possible to implement updates of the mainroad theme easily and also easily upadting my base customizations in all of my projects. But at the same time I could habe speicfic customizations for every project.
Is something like this possible with hugo?

Not sure I understand your goal, but you can nest themes as deep as you want.

Thanks, I found a way to implement the multiple nesting.
I had to put the mainroad theme into my projects root theme folder same as my my_base_customized_theme. Then I added a theme = "mainroad" into the config.toml of my customized theme. So now my structure is the following:

themes/my_base_customized_theme/config.toml (content: theme = "mainroad")
/config.toml (content: theme = "my_base_customized_theme")

My mistake was trying to put the original mainroad theme into a second theme directory of my customized theme.