Theme.js

function Theme() {}

Theme.prototype.createStylesheet = function() {
  return {
    header: {
      color: '#333',
      fontStyle: 'italic',
      fontFamily: 'Roboto, sans-serif',
    },
    background: {
      backgroundColor: '#fff',
    },
    button: {
      backgroundColor: '#fff',
      color: '#333',
    },
    color: '#fff',
  }
}

Theme.prototype.applyStylesheet = function(stylesheet) {
  const bodyElem = document.querySelector('body')
  const headerElem = document.getElementById('header')
  const buttonElems = document.querySelectorAll('button')
  this.applyStyles(bodyElem, stylesheet.background)
  this.applyStyles(headerElem, stylesheet.header)
  
  buttonElems.forEach((buttonElem) => {
    this.applyStyles(buttonElem, stylesheet.button)
  })
}

Theme.prototype.applyStyles = function(elem, styles) {
  for (let key in styles) {
    if (styles.hasOwnProperty(key)) {
      elem.style[key] = styles[key]
    }
  }
}