It's not too difficult to change a widget's font. Althought this isn't tested... if I recall, it's as easy as:
Code:
GtkWidget *w;
PangoFontDescription *font_desc;
/*
More code here to get a widget for w. Could be a GtkLabel, a GtkButton,
a GtkTextView, etc.
*/
/*
we specify the font in string format such as the string returned from
a call to gtk_font_selection_get_font_name()
*/
font_desc = pango_font_description_from_string ("monospace 10");
gtk_widget_modify_font (w, font_desc);
I love GTK+ as well.