Variable $XDG_CONFIG_HOME is usually set to ~/.config
${XDG_CONFIG_HOME}/gtk-3.0/settings.ini
[Settings]
gtk-theme-name = Adwaita
gtk-fallback-icon-theme = gnome
# next option is applicable only if selected theme supports it
gtk-application-prefer-dark-theme = true
# set font name and dimension
gtk-font-name = Sans 10