Aside from developing a custom plugin to handle this, it's not that easy and ChatGPT needs to have full view (context) of your codebase to recommend proper working code ie. trained on your codebase. If you make changes to the core files, you'll be running a custom, complex setup which will require maintaining a different codebase from the source.
It's doable if you know Php and have familiarized yourself with the codebase as there are lots of dependencies and many files to modify to achieve a multi-themed sub-domain setup.