In this way the background doesn't apply to the full main hbox, at the
current state this doesn't change much, but I guess it might in case of
semi-transparent backgrounds.

The hbox that contains the terminal screen and the scrollbar should have
the same background color of the terminal screen.

And relative getters.

https://bugzilla.gnome.org/show_bug.cgi?id=754254

CSS file is still empty, and the code works without it present,
so don't include it in the resources.