//gcc -Wall TestStyleChange.c -o TestStyleChange `pkg-config --cflags --libs gtk+-3.0`
//Tested on debian 3.16.0 and GTK3.14.5

#include<gtk/gtk.h>
void set_background_color(GtkWidget *w, gchar *color)
{
GtkCssProvider *gcp;
GtkStyleContext *gsc;
gsc = gtk_widget_get_style_context(w);
const gchar *type = g_type_name (G_TYPE_FROM_INSTANCE (w));
gchar *str = g_strdup_printf ("%s {background-color: %s;}", type, color);
gcp= gtk_css_provider_new();
gtk_css_provider_load_from_data(gcp, str, -1, 0);
g_free (str);
gtk_style_context_add_provider(gsc, GTK_STYLE_PROVIDER(gcp), 
    GTK_STYLE_PROVIDER_PRIORITY_APPLICATION);
}

void set_foreground_color(GtkWidget *w, gchar *color)
{
GtkCssProvider *gcp;
GtkStyleContext *gsc;
gsc = gtk_widget_get_style_context(w);
const gchar *type = g_type_name (G_TYPE_FROM_INSTANCE (w));
gchar *str = g_strdup_printf ("%s {color: %s;}", type, color);
gcp= gtk_css_provider_new();
gtk_css_provider_load_from_data(gcp, str, -1, 0);
g_free (str);
gtk_style_context_add_provider(gsc, GTK_STYLE_PROVIDER(gcp), 
    GTK_STYLE_PROVIDER_PRIORITY_APPLICATION);
}

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

   GtkWidget *top_window=gtk_window_new(GTK_WINDOW_TOPLEVEL);
   g_signal_connect (top_window, "destroy", G_CALLBACK(gtk_main_quit), NULL);
   
   GtkWidget *eventbox = gtk_event_box_new ();
   GtkWidget *label=gtk_label_new("Label");
   
   set_background_color (label, "#FFFF00");
   set_foreground_color (label, "#FF00FF");
  
   
   gtk_widget_set_size_request(label, 400, 400);
   gtk_container_add (GTK_CONTAINER (eventbox), label);
   gtk_container_add (GTK_CONTAINER(top_window), eventbox);
   gtk_widget_show_all (top_window);

   
   gtk_main();

   return 0;  
 }