#include <gtk/gtk.h>

GtkWidget* create_entry() {
  GtkWidget *entry = gtk_entry_new();

  /* style this entry. */
  /* GTK predefined CSS class for entry widget. All themes provide this class */
  gtk_style_context_add_class(gtk_widget_get_style_context(entry), "error");

  return entry;
}

void init() {
  GtkWidget *window = gtk_window_new(GTK_WINDOW_TOPLEVEL);
  GtkWidget *entry = create_entry();

  gtk_container_add(GTK_CONTAINER(window), entry);
  gtk_widget_show_all(window);

  g_signal_connect(window, "destroy", gtk_main_quit, NULL);
}

int main(int argc, char **argv) {
  gtk_init(&argc, &argv);

  init();
  gtk_main();
  return 0;
}