@julian might be wrong, but im guessing he is referring to user and theme integration. He said he has an existing site and he wants to integrate it more seamlessly, just a guess that is what he is referring to.

@jvandellen these might be what your looking for:
https://docs.nodebb.org/en/latest/themes/create.html
https://docs.nodebb.org/en/latest/plugins/create.html