how to change font of a radio button

[Jiang XU <jiang xu echostar com>]

I can change font and front color of other widgets, such as button and
label.
But I can not change the font of the radio button (GtkRadioButton).
I wonder why?

I have asked this question before, somebody told me to get the child
label of the radio button, but how I can get that child label?

If someone knows, please teach me.

Thanks