fuction2.html

<!DOCTYPE html>
<html lang="en">
  <head>
    <title>Fuction2</title>
    <script>
        function LinkssetColor(color){
            var alist = document.querySelectorAll('a');
                var i = 0;
                while(i < alist.length){
                    alist[i].style.color = color;
                    i = i + 1;
                }
        }
        function nightDayhandler(self){
            var target = document.querySelector('body');
            if(self.value === 'night'){
                target.style.backgroundColor = 'black';
                target.style.color = 'white';
                self.value = 'day';

                LinkssetColor('powderblue');
            } else {
                target.style.backgroundColor = 'white';
                target.style.color = 'black';
                self.value = 'night';

                LinkssetColor('blue');
            }
        }   
    </script>
  </head>
  <body>
    <h1><a href="index.html">신년계획</a></h1>
    <input
      id="night_day"
      type="button"
      value="night"
      onclick="nightDayhandler(this);">

    <ol>
      <li><a href="JS.html">HTML</a></li>
      <li><a href="JS2.html">CSS</a></li>
      <li><a href="JS3.html">JavaScript</a></li>
    </ol>
  </body>
</html>