There are a couple of ways of doing this. As GTK+ allows
the appearance of applications to be changed at run time using
resources you can use something like the following in the
appropriate file:
| style "text"
{
  font = "-adobe-helvetica-medium-r-normal--*-100-*-*-*-*-*-*"
} | 
Another way to do this is to load a font within your
program, and then use this in the functions for adding text to
the text widget. You can load a font using, for example:
|   GdkFont *font;
  font = gdk_font_load("-adobe-helvetica-medium-r-normal--*-140-*-*-*-*-*-*"); |