Here’s the situation:

I’ve created a child theme that enhances a popular parent theme. The enhancements include CSS, JS, and php. I want to use this child theme on multiple sites, and I want to allow for additional customization on each site without editing the child theme I created.

A few requirements:

• Each site uses the same base theme and a standard set of customizations (my child theme), but each site can have its own additional customizations (CSS, JS, and php).
• I don’t want to create my own base theme because I want to avoid that level of maintenance (security updates, compatibility, etc).
• I’d like to avoid customizing the child theme for each site. That way I can continue to develop my child theme and easily deploy updates without overwriting site-specific customizations.

My current solution is to create a directory outside the child theme that includes the CSS, JS, and php files for customizations, and then reference those in the child theme. But this is non-standard at best and possibly a security risk.

So what would be the best approach for this situation?

Edit: I don’t now ahead of time what customizations are needed. So the system needs to be very flexible and open-ended.