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;
}