Edit in JSFiddle

var chordColors = {A:{r:204,g:204,b:238},"A#":{r:204,g:204,b:255},B:{r:204,g:238,b:204},C:{r:204,g:238,b:238},"C#":{r:204,g:255,b:255},D:{r:238,g:204,b:204},"D#":{r:255,g:204,b:204},E:{r:238,g:204,b:238},F:{r:238,g:238,b:204},"F#":{r:255,g:255,b:204},G:{r:238,g:238,b:238},"G#":{r:255,g:255,b:255},N:{r:255,g:255,b:255}};

window.addEventListener('load', function() {
  var songleWidgetElement =
    SongleWidgetAPI.createSongleWidgetElement({
      api: "songle-widget-api-example",
      url: "www.youtube.com/watch?v=yIQDGwyJpSo",
      songAutoPlay: '0',
      songleWidgetSizeW: 'auto',
      videoPlayerSizeW: 'auto',
      videoPlayerSizeH: 'auto',
    });

  document.body.appendChild(songleWidgetElement);
});

// Songle Widget の準備が完了すると window.onSongleWidgetReady が実行される
window.onSongleWidgetReady = function(apiKey, songleWidget) {
  var el = songleWidget.rootElement;

  // ビートに合わせてプレーヤを脈動させる
  songleWidget.on("beatPlay", function(event) {
    el.setAttribute('class', 'beat-' + event.beat.position);
  });

  // コード進行に応じて背景色を変える
  songleWidget.on("chordEnter", function(event) {
    var color = chordColors[event.chord.name.match(/^([A-GN#]{1,2})/)[1]];

    if (color) {
      document.body.style.backgroundColor = 'rgb(' +
        [color.r, color.g, color.b].join(',') +
        ')';
    }
  });
};

<script src="http://widget.songle.jp/v1/api.js"></script>

@keyframes first-beat {
    0% { transform: scale( 1.4 ); }
    20% { transform: scale( .75 ); }
    50% { transform: scale( 1 ); }
}

@keyframes other-beat-2 {
    0% { transform: scale( 1.1 ); }
    20% { transform: scale( .85 ); }
    50% { transform: scale( 1 ); }
}

@keyframes other-beat-3 {
    0% { transform: scale( 1.1 ); }
    20% { transform: scale( .85 ); }
    50% { transform: scale( 1 ); }
}

@keyframes other-beat-4 {
    0% { transform: scale( 1.1 ); }
    20% { transform: scale( .85 ); }
    50% { transform: scale( 1 ); }
}

div.beat-1 {
    animation: first-beat 0.5s;
}

div.beat-2 {
    animation: other-beat-2 0.5s
}

div.beat-3 {
    animation: other-beat-3 0.5s
}

div.beat-4 {
    animation: other-beat-4 0.5s
}