mouse: Fix the look of test button

https://bugzilla.gnome.org/show_bug.cgi?id=703769

panels/mouse/cc-mouse-panel.c

@@ -98,6 +98,8 @@ cc_mouse_panel_constructed (GObject *object)
   box = gtk_box_new (GTK_ORIENTATION_HORIZONTAL, 2);
 
   button = gtk_button_new_with_mnemonic (_("Test Your _Settings"));
+  gtk_style_context_add_class (gtk_widget_get_style_context (button),
+                               "text-button");
   gtk_box_pack_start (GTK_BOX (box), button, FALSE, FALSE, 0);
   gtk_widget_set_visible (button, TRUE);