Retro scrollbars for Ubuntu (GTK3) programs, such as Firefox

Most modern applications have followed the trend of resizing scrollbars to the point of invisibility. To fix this for Firefox and other GTK3 programs, do the following:

Navigate to the following directory:

~/.config/gtk-3.0

Create the following file:

gtk.css

And add your custom CSS:

scrollbar, scrollbar button, scrollbar slider {
    -GtkScrollbar-has-backward-stepper: true;
    -GtkScrollbar-has-forward-stepper: true;
    min-width: 12px;
    min-height: 12px;
    border-radius: 0;
    border: 2px solid #777777;
}
scrollbar slider {
    border: 2px solid #333333;
    background: #151515;
}
scrollbar slider:hover {
    background: #333333;
    background: #151515;
}

Restart application to see the changes. The end result looks like this:

Adjust the colours and sizes as you see fit.