Substitutin fonts with ~/.fonts.conf

[Matteo Riva <muradin@gmail.com>]

I'm trying using ~/.fonts.conf to replace Helvetica with Droid Sans,
here's the content of the file:

<?xml version="1.0"?>
<!DOCTYPE fontconfig SYSTEM "fonts.dtd">
<fontconfig>
 <match target="pattern">
  <test qual="any" name="family"><string>Helvetica</string></test>
  <edit name="family" mode="assign"><string>Droid Sans</string></edit>
 </match>
</fontconfig>

but no matter what, nothing seems to happen.  I tried running fc-cache,
I have checked in /etc/fonts/conf.d and I found 50-user.conf which seems
to explicitly call user specific configurations.

What am I missing?

Thanks