Hide the scrollbar from the window manager

Question

My system:
Beaglebone Black,
Debian Stretch V9.9,
5" Display (Matrix Orbital)

browser: Midori V.0.5.11
additionally I have installed the following
openbox,
xserver-xorg-video-fbdev,
x11-xserver-utils,
lightdm

I wrote a web app in html5 and node express.

I am using the window manager to display the web app in kiosk mode.
$ DISPLAY=:0 midori -e Fullscreen -a http://localhost:8082

My problem:
The vertical scrollbar is probably displayed very thin by the window manager. I want to make this scrollbar wider or hide it and use CSS to create my own scrollbar.

If I create a custom scrollbar myself I get 2 scrollbars.

/* width */
::-webkit-scrollbar {
  width: 20px;
}

/* Track */
::-webkit-scrollbar-track {
  box-shadow: inset 0 0 5px grey;
  border radius: 10px;
}

/* Handle */
::-webkit-scrollbar-thumb {
  background: red;
  border radius: 10px;
}

/* Handle on hover */
::-webkit-scrollbar-thumb:hover {
  background: #b30000;
}

Who can help me ? please have a look at the photos which explain more than 1000 words.

one_small_scrollbar two scrollbars,css and window manager

Answer 1

My solution:
$ sudo apt install gtk-theme-switch
$ DISPLAY=:0 gtk-theme-switch2 -d
select the theme "Adwaita"

/usr/share/theme/Adwaita/gtk-2.0/main.rc
GtkRange::slider-width = 13
With this variable you can set the width.

Ready