FVWM: My new mouse cursor works well except on rootwindow

[for.register for.register]

Hi,

I have download a X11 mouse cursor theme and put it into ~/.icons.
It used to work well in Fluxbox, but in Fvwm, there is a little
problem, the cursor works within opened app window, but changes
back to the native X11 default cursor in rootwindow. I don't
know if that is a defect of the new mouse cursor them or I should
do something on the Fvwm configuration.

Thanks.