GTK CSS issue reproducer

sample.c

/*
 * gcc $(pkg-config --cflags gtk+-3.0) sample.c -o sample $(pkg-config --libs gtk+-3.0)
 */

#include <gtk/gtk.h>

int main (int argc, char *argv[])
{
  GtkWidget *window;
  GtkWidget *box;
  GdkRGBA    color;
  GtkCssProvider *provider;

  gtk_init (&argc, &argv);

  provider = gtk_css_provider_new();
  gtk_css_provider_load_from_data(provider,
                                  "#xfwm-tabwin { "
                                    "border-radius: 100px;"
                                    "box-shadow: 10px 10px 5px 0px rgba(0,0,0,0.75);"
                                  "}", -1, NULL);
  gtk_style_context_add_provider_for_screen (gdk_screen_get_default (),
                                              GTK_STYLE_PROVIDER(provider),
                                              GTK_STYLE_PROVIDER_PRIORITY_FALLBACK);

  window = gtk_window_new (GTK_WINDOW_TOPLEVEL);

  gtk_window_set_position (GTK_WINDOW (window), GTK_WIN_POS_CENTER);
  gtk_container_set_border_width (GTK_CONTAINER (window), 5);
  gtk_window_set_default_size (GTK_WINDOW (window), 640, 480);

  gdk_rgba_parse (&color, "#173F5F");
G_GNUC_BEGIN_IGNORE_DEPRECATIONS
  gtk_widget_override_background_color (window, GTK_STATE_FLAG_NORMAL, &color);
G_GNUC_END_IGNORE_DEPRECATIONS

  box = gtk_box_new (GTK_ORIENTATION_HORIZONTAL, 0);

  gtk_widget_set_valign (box, GTK_ALIGN_CENTER);
  gtk_widget_set_halign (box, GTK_ALIGN_CENTER);
  gtk_widget_set_size_request (box, 200, 200);
  gdk_rgba_parse (&color, "#3CAEA3");
G_GNUC_BEGIN_IGNORE_DEPRECATIONS
  gtk_widget_override_background_color (box, GTK_STATE_FLAG_NORMAL, &color);
G_GNUC_END_IGNORE_DEPRECATIONS
  gtk_widget_set_name (GTK_WIDGET (box), "xfwm-tabwin");
  gtk_container_add (GTK_CONTAINER (window), box);

  g_signal_connect (G_OBJECT (window), "destroy", G_CALLBACK (gtk_main_quit), NULL);

  gtk_widget_show_all (window);

  gtk_main ();

  return 0;
}