Nordic Polar is a Gtk3.20+ theme created using the awesome Nord color palette.
Extract the zip file to the themes directory i.e. /usr/share/themes/ or ~/.themes/ (create it if necessary).
To set the theme on Gnome, run the following commands in Terminal:
gsettings set org.gnome.desktop.interface gtk-theme "Nordic-Polar"
gsettings set org.gnome.desktop.wm.preferences theme "Nordic-Polar"
or Change via distribution specific tool.