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The default style is determined by the Theme the user has selected. However, the GtkLabel widget has functions to apply Pango markup directly which you CAN do in glade. First, you have to change the 'Use Markup' property to 'TRUE' and then you can put markup right into the label's text property. For example, you could do '<span size="larger">This is a label</span>' or '<span font="Monospace 10.5">This is a label</span>'.
To see a list of available markup, refer to the Pango manual:
http://library.gnome.org/devel/pango...kupFormat.html
In addition to doing this in Glade, you can do it in code as well using gtk_label_set_use_markup() and gtk_label_set_markup().