background color fishing

main.js

(function () {
'use strict';

function getRgbArray(rgb) {
  return rgb.match(/[0-9]+/g);
}

function getDarkenedRgbArray(rgb) {
  let rgbs = getRgbArray(rgb);
  return [Math.round(rgbs[0] / 2), Math.round(rgbs[1] / 2), Math.round(rgbs[2] / 2)];
}

function getRgbTotal(rgb) {
  let rgbs = getRgbArray(rgb);
  return parseInt(rgbs[0], 10) + 
    parseInt(rgbs[1], 10) + 
    parseInt(rgbs[2], 10);
}

for (let el of [].slice.call(document.querySelectorAll('*'))) {
  let bg = el.style.backgroundColor;
  if (bg) {
    if (getRgbTotal(bg) > 255 * 3 / 2) {
      let rgbs = getDarkenedRgbArray(bg);
      let newBg = `rgb(${rgbs[0]}, ${rgbs[1]}, ${rgbs[2]})`;
      console.log(newBg);
      el.style.backgroundColor = newBg;
    }
  } else {
    el.style.backgroundColor = '#555';
  }
}

}())