Tip: Fix that shut down icon in the top-right corner (for the Humanity “Ambiance” theme)
sudo cp '/usr/share/icons/Ambiant-MATE/actions/24/system-shutdown.svg' '/usr/share/icons/Humanity/actions/24/system-shutdown.svg'
I use to tweak my appearance to the Ambiance orange theme by switching the icons to Ubuntu-Mono-Dark and changing all the green colours I could find (some traces are harder to change). Somebody could whip up an archive containing all the changes.
I’ve recently switched to the Ambiance-Colours themes.