change the Ubuntu system background color

sudo -i gedit /usr/share/themes/Ambiance/gtk-2.0/gtkrc

tooltip_fg_color:提示框前景色 (改为：#000000)

tooltip_bg_color:提示框背景色 (改为：#f2edbc)

line1: base_color:#C7EDCC (eye-protecting green)

then change the theme to Ambiance

system settings -> appearance -> theme