(For Clearlooks-Phenix users)
On Ubuntu 15.10 and 16.04, gnome-tweak-tool doesn’t provide the item for window theme selection. Alternatively, you can apply Clearlooks-Phenix theme to window title bars in GNOME3 flashback sessions (for both metacity and compiz) by typing:
$ gsettings set org.gnome.metacity theme Clearlooks-Phenix