Buttons Don't Change Color

//PLAYLIST.JS
function Playlist() {
  this.songs = [];
  this.nowPlayingIndex = 0; 
}

Playlist.prototype.add = function(song) {
  this.songs.push(song);
};

Playlist.prototype.play = function() {
  var currentSong = this.songs[this.nowPlayingIndex]; 
  currentSong.play(); 
};

Playlist.prototype.stop = function(){
  var currentSong = this.songs[this.nowPlayingIndex]; 
  currentSong.stop(); 
};

Playlist.prototype.next = function() {
  this.stop();
  this.nowPlayingIndex++; 
  if(this.nowPlayingIndex === this.songs.length) {
    this.nowPlayingIndex = 0; 
  }
  this.play();
};

Playlist.prototype.renderInElement = function(list) {
  list.innerHTML = ""; 
  for (var i = 0; i < this.songs.length; i++) {
    list.innerHTML += this.songs[i].toHTML(); 
  }
};

//SONG.JS
function Song(title, artist, duration) {
  this.title = title;
  this.artist = artist;
  this.duration = duration;
  this.isPlaying = false;
}

Song.prototype.play = function() {
  this.isPlaying = true;
};

Song.prototype.stop = function() {
  this.isPlaying = false;
};

Song.prototype.toHTML = function() {
  var htmlString = '<li';
  if(this.isPlaying === true) {
  htmlString += 'class="current"';
  }
  htmlString += '>';
  htmlString += this.title;
  htmlString += ' - ';
  htmlString += this.artist;
  htmlString += '<span class="duration">';
  htmlString += this.duration;
  htmlString += '</span></li>';
  return htmlString;
};

//APP.JS
var playlist = new Playlist(); 

var hereComesTheSun = new Song("Here Comes the Sun", "The Beatles", "2:54");
var walkingOnSunshine = new Song("Walking on Sunshine", "Katrina and the Waves", "3:43"); 

playlist.add(hereComesTheSun);
playlist.add(walkingOnSunshine); 

var playlistElement = document.getElementById("playlist"); 

playlist.renderInElement(playlistElement); 

var playButton = document.getElementById("play"); 
 playButton.onclick = function () {
  playlist.play(); 
  playlist.renderInElement(playlistElement); 
}

var nextButton = document.getElementById("next"); 
nextButton.onclick = function () {
  playlist.next(); 
  playlist.renderInElement(playlistElement); 
}

var stopButton = document.getElementById("stop"); 
stopButton.onclick = function () {
  playlist.stop(); 
  playlist.renderInElement(playlistElement); 
}