Here the sample code of how to overwrite draw_expander. You'll definetely have to take a look in the manual to get all the parameters right.
#include <gtk/gtk.h>
#include <cairo.h>
enum {
COL_1,
N_COLS
};
void draw_expander (GtkStyle *style,
GdkWindow *window,
GtkStateType state_type,
GdkRectangle *area,
GtkWidget *widget,
const gchar *detail,
gint x,
gint y,
GtkExpanderStyle expander_style) {
cairo_t *cr;
cr = gdk_cairo_create (window);
cairo_set_source_rgb(cr, 0, 0, 0);
cairo_move_to (cr, 0, 0);
cairo_line_to (cr, 0, 10);
cairo_line_to (cr, 10, 5);
cairo_close_path (cr);
cairo_stroke (cr);
}
GtkWidget *build_view (); /* just supply your own */
int main (int argc, char *argv[]) {
gtk_init (&argc, &argv);
GtkWidget *window;
GtkWidget *view;
window = g_object_new (GTK_TYPE_WINDOW, NULL);
view = build_view ();
gtk_container_add (GTK_CONTAINER (window), view);
GtkStyle *style = gtk_widget_get_style (view);
GtkStyleClass *klass = GTK_STYLE_GET_CLASS (style);
klass->draw_expander = draw_expander;
gtk_widget_show_all (window);
gtk_main ();
return 0;
}