options.js contains:

------

'use strict';

const kButtonColors = ['#3aa757', '#e8453c', '#f9bb2d', '#4688f1'];

function constructOptions(kButtonColors) {
  for (let item of kButtonColors) {
    let button = document.createElement('button');
    button.style.backgroundColor = item;
    button.addEventListener('click', function() {
      chrome.storage.sync.set({color: item}, function() {
        console.log('color is ' + item);
      })
    });
    page.appendChild(button);
  }
}
constructOptions(kButtonColors);

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But looks like page is never defined, so the end result is that the page looks empty (and throws an error).