Config file locations:

• ~/.gtkrc-2.0
• ~/.config/gtk-3.0/settings.ini

I hate the scrollbars in GTK3, they're really narrow, and hide when you're not hovering over them. To fix them I set the environment variable GTK_OVERLAY_SCROLLING=0 and add this CSS to ~/.config/gtk-3.0/gtk.css

undershoot.top,
undershoot.right,
undershoot.bottom,
undershoot.left {
	background-image:none;
}

• Source: https://slackalaxy.com/2017/11/27/gtk3-overlay-scrollbars/

• GTK