decrease Adwaita theme border

[sabayon11 <sabayon11 gmail com>]

How can I decrease Adwaita theme border width? I use Plasma KDE in Ubuntu
18.04 but for gtk2 and gtk3 apps I use Adwaita. The problem is that right border
width is too thick and when I move mouse to the right side of the
screen mouse cursor doesn't touch scrollbar and it is impossible to
scroll. I need to make right border thinner. And I would prefer to set
it in pixels if possible.

I found those guidelines but I need to know the exact component that
need to be changed and what values should I use.

https://developer.gnome.org/gtk2/stable/GtkWindow.html

https://developer.gnome.org/gtk3/3.5/GtkWindow.html

I prefer to do it directly in theme because I don't want to install
additional Gnome tools.
Is it possible?