Scrollbar very wide in all GTK applications

I’ve had this problem for a very long time but now I thought I should check with the community how to solve the problem.
In several applications, that I thing uses GTK gives a very wide scrollbar, e.g. qpdfviewer, MasterPDF, games.

I’ve tried to set the config file ~/.config/gtk-3.0/gtk.css and adding the text:

scrollbar, scrollbar button, scrollbar slider { min-width: 10px; min-height: 10px; }

but it did not make any difference.
Any ideas?

it’s not a gtk problem, it’s qt.