var pageSettings = {

  red: 200,
  green: 200,
  blue: 200

};
var bgColor = 'rgb(' + pageSettings.red + ', ' + pageSettings.green + ', ' + pageSettings.blue + ',' + pageSettings.background: 'https://pictures.s3.yandex.net/cover-wall.jpg');
document.body.style.backgroundColor = bgColor;
var header = document.getElementById('main-header');
console.log(header);