Feature Request: Provide post-hook after ColorScheme being applied

[nyngwang]

As title. I have a custom User event which will do a lot of things for fine-tuning all colorschemes I downloaded from GitHub to avoid glitch-like appearance when working with my existing plugins.

It will be great if you could either:

1. Define a custom autocmd event too after the AI-generated ColorScheme has been applied after it received a user's request.
2. Provide a hook-point at setup so that I can just put my callback(s) there.

What do you think?