script.js

(function($) {
  chrome.runtime.onMessage.addListener(function(msg, sender) {
    $.getElementById('kouho').value = msg.text;
  });
})(document);

// author: https://gist.github.com/yuru4c/0069ce5724db34c9085ffe2d51b23611
(function($) {
  var e = $.getElementsByClassName('schedule');
  var a = e[0].style,
    b = e[1].style;

  var s = $.createElement('button');
  s.onmouseover = function() {
    s.style.backgroundColor = 'transparent';
  };

  // adjust style of the chouseisan
  s.onmouseleave = function() {
    s.style.backgroundColor = '#EEEEEE';
  };
  s.onclick = function() {
    a.display = 'none';
    b.display = 'table';
    var t = a;
    a = b;
    b = t;
  };
  s.style.fontSize = '13px';
  s.style.backgroundColor = '#EEEEEE';
  s.style.border = '1px solid #CCCCCC';
  s.style.color = '#616161';
  s.style.borderRadius = '5px';
  s.style.outline = 'none';
  s.appendChild($.createTextNode('行と列を切り替え'));

  // select the first element after .event-choice-table-header-div.
  var h = $.querySelector('.event-choice-table-header-div *');
  h.appendChild($.createTextNode(' '));
  h.appendChild(s);
})(document);