How to change default global font in FC2

Hi,

I'd like to change the default global font used in the system.
In fact I want to use KDE and I hate the "Sans" font, which is not as clear as
"Verdana" for example.

I changed anything in the control center to use Verdana instead of Sans, but when I launch
applications as Mozilla, Firefox, Thunderbird, or Evolution, "Sans" is still
used, and I guess it's the default system font and I'd like to change it ...

In Gnome or XFCE, when changing the application font to Verdana, those apps above are
displaying well Verdana instead of Sans, so I guess Gnome and XFCE use something else than
KDE for this ...

How can I change that ?

Any help would be very appreciated,

thanks,

Bastien.