How to get Mac style title bar buttons on Ubuntu gnome without changing rest of the theme?

First you will need to find a theme with the buttons you want and install that theme.

You can find many OS X themes at gnome-look, search gtk3 theme in the type and osx in the description, though I will recommend you, based on your question, this nice flat Yosemite theme (Developers screenshot).

This should look really nice with the Arc theme, in my opinion.

Next install

sudo apt-get install gnome-tweak-tool

When it is finished start this app from the launcher or

gnome-tweak-tool

Browse to the Appearance section and you will see this

Change the window theme to change your window declarations and gtk3 for the rest of the theme.

I found a perfect theme that satisfies what I need -

Arc Flatabulous: https://github.com/andreisergiu98/arc-flatabulous-theme

There's also another one with not much differences on gnome-look called OSX-Arc, but the development of Arc-Flatabulous is more active than OSX-Arc

i'm using right now a theme from noobslab and they doing a great job, it makes everything to feel like osx!!

Here is the website http://www.noobslab.com/2016/04/macbuntu-1604-transformation-pack-for.html

if you follow the steps correctly you will have the same exactly result!!