I have tried to install GTK themes as well as window themes and I get problems in both cases depending on the various themes available.
To do so I copied the theme in
/usr/share/themes, and then selected the themes in
gnome-tweak-tool both as GTK themes and Window themes. In some cases (Zukitwo and Elementary) the window theme works but not the Gtk one, and in the case of Boomrang it is the window theme that fails, whilst Gtk works.
How can I successfully install any of those themes?