In case it is useful for others, this is a summary of the changes that I made from the default Zorin, which made scrollbars the correct width in most programs (including Firefox) regardless of theme chosen. There may be some apps such as Thunderbird which retain narrow bars. All this was thanks to the amazing Aravisian!
First Change
In your Home Directory, you have a folder : .config
If you do not see it, hit the keyboard shortcut ctrl+h to reveal the hidden folders. This shortcut toggles that on and off.
Shorthand, in Linux, for the path to your Home Directory (/home/$USER) is simply: ~
In this case, navigate to ~/.config/gtk-3.0/ and in that directory, create a new Text File named gtk.css
Paste the following into it:
scrollbar.overlay-indicator:not(.dragging):not(.hovering) slider,
scrollbar.overlay-indicator:not(.dragging):not(.hovering).horizontal slider, scrollbar.overlay-indicator:not(.dragging):not(.hovering) trough {
min-width: 8px;
min-height: 8px;
}
scrollbar trough, scrollbar slider {
min-width: 16px;
}
scrollbar.horizontal trough, scrollbar.horizontal slider {
min-height: 16px;
}
Save the file.
Log out and in or reboot to test.
Disable Overlay Scrollbars
In terminal run:
gsettings set org.gnome.desktop.interface overlay-scrolling false
You may also need to add the following to your ~/.profile file (can just put it at the end):
export GTK_OVERLAY_SCROLLING=0
Firefox change
If you are looking at the Scrollbars in Firefox browser... They will not use the gtk scrollbars (Your system theme) unless you tell it to.
Thunderbird is also a Mozilla product. It does the same as firefox.
In your browser address bar (in a new tab) type
about:config
There is a search bar - search "scrollbar"
Set widget.gtk.alt-theme.scrollbar to true
Set widget.gtk.alt-theme.scrollbar_active to true
Set widget.non-native-theme.enabled to false
Set widget.non-native-theme.gtk.scrollbar.round-thumb false
Set widget.non-native-theme.win.scrollbar.use-system-size false
Set widget.non-native-theme.scrollbar.size to 16
Set widget.non-native-theme.scrollbar.active-always-themed true
This should get everything working that uses gtk3.