This is a GNOME Shell theme designed to fit the default GNOME 3 style. The default GTK3 theme is called Adwaita and this theme tries to make them look consistent together.
Please avoid a manual install and use the GNOME Tweak Tool, it's much easier. You can use the gnome-shell theme extension for easier switching of themes.
GNOME Tweak Tool
Make sure you have user-theme-extension installed. Load the GNOME Tweak Tool and go to Shell Extensions and click the "Use Theme Extension" switch. Now go to Theme>Shell Theme and click the box and locate your theme zip. Now you can select the theme in the dropdown box.
Compatibility: GNOME 3.2