You will probably need to do rather large edits in these two files:
to match XCode's syntax highlighting. That's a non-trivial task that you can't really expect anybody here to do for you. The facts that a) those files are 4 years old and b) their author has completely ceased to support his Vim stuff don't make it any simpler.
I'm not aware of a single comprehensive online tutorial on writing your own syntax script so you won't be able to escape reading
:help syntax from top to bottom and back and taking a look at other syntax scripts in
FWIW, the highlight groups defined by cocoa.vim are all linked to these seven standard highlight groups (I've added their hex value taken from your colorscheme):
Error no value
You'll notice that, in base16,
Constant have the same colors which may not be a good thing. From your XCode screenshot, it doesn't seem that it uses more colors than base16 so adding a couple of syntax groups might be enough.