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

To install

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

Info

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