learning_jquery/Chapter 3/03.js

$(document).ready(function () {
  'use strict';
  $('#switcher-default').addClass('selected');
  
  $('#switcher button').on('click', function () {
    var bodyClass = this.id.split('-')[1];

    $('body').removeClass().addClass(bodyClass);

    $('#switcher button').removeClass('selected');
    $(this).addClass('selected');
  });
});