var setClass = function() {
  var a = this.className
  a = a.split(' ');
  a.push('nobg');
  a = a.uniq();
  this.className = a.join(' ');
}

Event.observe(window, 'load', function() {
  if (document.getElementById('extra-loginform')) {
    Event.observe('i_username', 'click', setClass);
    Event.observe('i_username', 'focus', setClass);
    Event.observe('i_password', 'click', setClass);
    Event.observe('i_password', 'focus', setClass);
  }
});