raijin: Here's my best attempt at a tutorial...
First download the GTK theme from https://www.gnome-look.org/p/1013714/
Select the Sierra-light one, its the first listed. Once downloaded, extract it and place it in the ~/.themes directory.
Second download the cursor theme from https://www.gnome-look.org/p/1084939/
Make sure to select the one that is OSX-ElCap, NOT THE KDE ONES. Once downloaded, extract it and open a terminal and run the install script that is located in the extracted folder.
Now at this point open the control center and open Appearances. Select the Sierra-light theme. Then select Customize and select the icon pack that you previously downloaded under the icon tab. Then select click the Pointer tab and select the OSX-ElCap cursor theme.
Now at this point you need to setup a dock. You can either use Plank that comes with Ubuntu MATE (probably the easiest), or download and install another dock. I use docky, but that's just because I was more familiar with it and like its ability to be customized. Open Mate Tweak and navigate to the panels tab. Select the cupertino layout which will provide you with a plank dock and a global menu on the top bar.
Now there a few additional things that I have added to my desktop to enable this "look," like a semi-transparent top bar, I'm using Compiz as my window manager, and I have a conky enabled as well. If you require assistance with those items I can point you in the right direction for those as well. Hope that this helped.