:-$ sudo apt-get install gnome-themes-standard
:-$ sudo add-apt-repository ppa:tualatrix/ppa
:-$ sudo apt-get update
:-$ sudo apt-get install ubuntu-tweak
:-$ ubuntu-tweak
-----------------------------------------
Then,
Ubuntu Tweak<Tweaks<Startup-Login Settings<Unlock<Gtk theme<Adwaita.
Close. Reboot.