gtk_widget_set_style and window

[Olivier Chambard <olivier chambard3 libertysurf fr>]

As written in Harlow's book, I use gtk_widget_set_style to change the
style of any existing widget.
It works all right but not with window, as shown in the following
example 
(The color changes as expected, but the window's size too with each
button-click)
Where is my undestanding?

/*
 *  compile with gcc -Wall -g pb.c -o pb `gtk-config --cflags`
`gtk-config --libs`
 */

#include <gtk/gtk.h>

GtkWidget *window, *button;
GtkStyle  *defStyle;	
GdkColor Color = {0, 0xffff, 0x0000, 0x0000};

/* prototypes */

gint WindowClicked (GtkWidget *widget, gpointer *data);
int main(int argc, char *argv[]);

int main(int argc, char *argv[]) {

  gtk_init(&argc, &argv); 

  defStyle = gtk_widget_get_default_style (); 
  gdk_color_alloc (gdk_colormap_get_system(), &Color);

  window = gtk_window_new(GTK_WINDOW_TOPLEVEL);
  gtk_widget_show (window);
  gtk_widget_set_usize(window, 300, 200);    
  gtk_window_set_position(GTK_WINDOW(window), GTK_WIN_POS_CENTER);
  gtk_container_set_border_width(GTK_CONTAINER(window), 50);
  
  button = gtk_button_new_with_label("bouton");
  gtk_container_add (GTK_CONTAINER(window), button);
  gtk_widget_show (button);
  gtk_signal_connect(GTK_OBJECT(button), "clicked",
GTK_SIGNAL_FUNC(WindowClicked), NULL);

  gtk_main();  
  return 0;                                                  
}

gint WindowClicked (GtkWidget *widget, gpointer *data)
{ 
  GtkStyle *windowstyle;

  windowstyle = gtk_style_copy (defStyle);

  if (Color.red ==  0xffff) {
    Color.red = 0x0000;
    Color.blue = 0xffff;
  }
  else {
    Color.red = 0xffff;
    Color.blue = 0x0000;
  }

  windowstyle->bg[GTK_STATE_NORMAL] = Color;
  defStyle = gtk_style_copy (windowstyle);
  gtk_widget_set_style (window, windowstyle);

  return (FALSE);
}