This is an example GTK theme, aims to be properly written and easily to expand for creating your own themes. It is based and supports the current development GTK+ version 3.22, but it should work at least on 3.20
You need Node 6.0, and you can get it from NVM
1. Clone the repo
git clone https://github.com/alex285/gnome-play-theme
2. Install dependencies
cd gnome-play-theme
npm install
3. Build it
gulp
That will create a CSS file
gnome-play-theme/dist/public/gtk/play-dark/gtk.css
4. Install it
Create a new GTK3 theme folder
sudo mkdir -p /usr/share/themes/Play/gtk-3.0
Copy Adwaita (or any other) GTK2 theme
sudo cp -R /usr/share/themes/Adwaita/{gtk-2.0,index.theme} /usr/share/themes/Play/
Copy the gtk.css file you built before
sudo cp -R dist/public/gtk/play-dark/gtk.css /usr/share/themes/Play/gtk-3.0/
DONE!
Select the theme (Play) with GNOME Tweak Tool
When you test themes, test them with GTK Inspector. So basically do not pick this theme from Tweak Tool, because it is totally unusable, and you may not even be able revert :p