How to remove button background shadows in GTK? (C) - c

Question

I'm new in the gtk's world and I cannot find a solution to this little problem.
I have this button :
But I would like that the small shadow behind the button can be removed.
I'd like this result :
How can I solve it?
Button's code:
sum_button = gtk_button_new_from_stock(GTK_STOCK_ADD);
/**/
style = gtk_widget_get_style(sum_button);
style->bg[GTK_STATE_PRELIGHT] = style->bg[GTK_STATE_NORMAL];
gtk_widget_set_style(sum_button, style);
/**/
gtk_widget_modify_bg(sum_button, GTK_STATE_NORMAL, &color2);
gtk_button_set_relief(GTK_BUTTON(sum_button), GTK_RELIEF_HALF);
g_signal_connect(G_OBJECT(sum_button), "clicked", G_CALLBACK(PrintNumber),&t_data);
gtk_box_pack_start(GTK_BOX (hbox3), sum_button, TRUE, TRUE, 0);

Answer 1

Try
gtk_button_set_relief(GTK_BUTTON(sum_button), GTK_RELIEF_NONE);
But then you can not set the background color:
When the button's border relief is set to none, it acts like a label as in it is "transparent", it has the same bg color as its parent container.
Take a look to this thread: https://stackoverflow.com/a/1709648/1606345

Answer 2

If you are writing new code, consider using GTK 3, where you can do this with CSS directly:
GtkCssProvider *provider = gtk_css_provider_new();
gtk_css_provider_load_from_data(provider,
"button {"
" border: 0px;"
" background-color: green;"
" color: white;"
"}", -1, &error);
GtkStyleContext *context = gtk_widget_get_style_context(button);
gtk_style_context_add_provider(context, provider,
GTK_STYLE_PROVIDER_PRIORITY_USER);