On Linux, I was able to solve this by creating a file called "fontsettings" with the following content:
style "small" {
font_name = "Sans Condensed 8"
}
class "GtkWidget" style "small"
Now I have to
export GTK2_RC_FILES=$GTK2_RC_FILES:./fontsettings
and then launch Arduino. Of course you can integrate the export statement into the Arduino launch script.