I was curious about this too. The source is on GitHub (https://github.com/350d/stylish), but who knows if Apple checks that they're the same when they're approving it.
Edit: I should note that it collects analytics, but it can be turned off in the preferences. I don't remember if it's on by default, but I suspect it is.
I really love that one, it does a great job in Safari. Unfortunately, there is no Safari App Extension yet. Since I'm running Safari Preview and Safari 12 does not accept extensions from unknown sources anymore I'm out for now.
Edit: I should note that it collects analytics, but it can be turned off in the preferences. I don't remember if it's on by default, but I suspect it is.