Click button to change CSS in D7
Your problem resides in the way you are calling the css function in jQuery. Try this, it works for me:

(function($) {
  Drupal.behaviors.my_node_toggle = {
    attach: function(context, settings) {
      $("#change_css", context).click(function() {
        $('a').css("color","#ff0");
      });
    }
  };
})(jQuery);