Edit for the theme Ambiance (for example):

• /usr/share/themes/Ambiance/gtk-3.0/settings.ini
• /usr/share/themes/Ambiance/gtk-3.0/gtk.css
• /usr/share/themes/Ambiance/gtk-2.0/gtkrc

• #000000 - tooltip_bg_color
• #f5f5b5 - tooltip_fg_color