Large frames on Gtk3

[Massimo-B]

On Gtk+ 3.22.30 and using the https://code.launchpad.net/~gnome-terminator/terminator/gtk3 port of Terminator, the splitters of horizontally and vertically split terminals are big, even when setting splitter size to 0 or 1, which is tiny on other themes.
It's also visible in Xfce's panel items like System Load or Network Monitor, the bars are huge.