Custom JS that styles Scrollbars in Firefox to resemble a grey ClearLooks Gtk theme.
Extra steps are necessary to get Firefox to load custom JS, e.g. methods 1 or 2 documented at https://github.com/Aris-t2/CustomJSforFx.
As of Fx113, scrollbar width CSS doesn't work, so
widget.non-native-theme.scrollbar.size.override needs to be
configured to cs_width + 2x cs_border! (16 if you kept the defaults.)
For best results, widget.non-native-theme.gtk.scrollbar.allow-buttons and
the 'always show scrollbars' setting should both be true.