Change Window Opacity / Transparency

Is there a way to set transparency to Ubuntu windows, without using extensions?

I found a command which does it, but only for the current session. Maybe someone can tweak it to be permanent?

sh -c 'xprop -f _NET_WM_WINDOW_OPACITY 32c -set _NET_WM_WINDOW_OPACITY $(printf 0x%x $((0xffffffff * 90 / 100)))'

It will activate a cursor to click/point over a window to add the values. Also it will do different colours, but it is only for the current session. After reboot it will be back to defaults.