For change this icon, I have modified this icon "/usr/share/icons/ubuntu-mono-dark/actions/16/system-shutdown-panel.svg" (see the attachment) and I copy the new icon in "/usr/share/icons/Humanity/actions/32/system-shutdown.svg" (32 because I have modify the default size panel to 32 ; you can use the other size 128 16 22 24 32 48).
For change this icon, I have modified this icon "/usr/share/ icons/ubuntu- mono-dark/ actions/ 16/system- shutdown- panel.svg" (see the attachment) and I copy the new icon in "/usr/share/ icons/Humanity/ actions/ 32/system- shutdown. svg" (32 because I have modify the default size panel to 32 ; you can use the other size 128 16 22 24 32 48).
So, I have a good icon for the shutdown applet.