Question about GtkScrollbar class in custom themes

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http://askubuntu.com – I very much like to use the Zukitwo theme as an alternative over the standard Ambiance theme, it is to my taste more easy on the eyes. However, the theme takes away the scrollbar arrows or "steppers" (which are normally part of the scrollbars in e.g. thunderbird and firefox in the Ambiance theme). I found this information: https://live.gnome.org/GnomeArt/Tutorials/GtkThemes/GtkScrollbar and was looking around in the gtk stylesheets of the theme: /usr/share/themes/Zukitwo-Dark/gtk-3.0/gtk-widgets.css but I have no experience at all in editing themes to get the arrows back in place again. (HowTos)