MacOS – Espresso Libre theme for MacVim

Are you sourcing your .vimrc.local by hand? That is not a standard file. MacVIM will read your ~/.vimrc and ~/.gvimrc. I'd suggest you keep your syntax settings in .gvimrc for MacVIM unless you want to use them in your terminal as well.

If you want to keep a generic .gvimrc that is shared by multiple systems and set your color only for this machine in .gvimrc.local you can put the following at the end of your ~/.gvimrc

" Allow for local machine-specific modifications
if filereadable($HOME . "/.gvimrc.local")
  source ~/.gvimrc.local
endif 

~ always is your home folder (/Users/rororo for instance). If ~/.vim exists, that's where you'll find it. Unless you've used vim before, the folder likely will not already exist, so to create the folder, open Terminal.app and type

cd ~
mkdir .vim

Assuming you downloaded ColorSamplerPack.zip to your Downloads folder you can then proceed with

cd .vim
unzip ~/Downloads/ColorSamplerPack.zip

to unpack the content into the folder. A restart of gvim may be required afterwards.

On second thought, the same result can be accomplished by just opening Terminal.app and executing

unzip ~/Downloads/ColorSamplerPack.zip -d ~/.vim