Customize scrollbar width for Firefox and other GTK3 apps in Linux

Hallelujah! The ridiculously thin vertical scrollbars with Firefox, Thunderbird, and any other Linux apps that use GTK3 for their window toolkit has annoyed me for sooo long. But I just found this simple, excellent fix, courtesy of this post at superuser.com.

Create ~/.config/gtk-3.0/gtk.css if it doesn’t exist and add the following contents:

scrollbar, scrollbar button, scrollbar slider {
    -GtkScrollbar-has-backward-stepper: true;
    -GtkScrollbar-has-forward-stepper: true;
    min-width: 20px;
    min-height: 20px;
    border-radius: 0;
}

Change min-width and min-height to whatever value you want the scrollbar width to be.

This will override theme-specific scrollbar width settings, so you never have to worry again about the scrollbars becoming ridiculously thin after changing the GTK3 theme!