Starter Skeleton

Here is a minimal, yet complete example of HTML, CSS and Javascript. Live demo of this on Glitch

Save the contents of each into a file (thus you should have three files: index.html, style.css and script.js) and then open index.html in your browser.

HTML (index.html):

<html>
  <head>
    <meta charset="utf-8" />
    <title>My page</title>
    <link rel="stylesheet" href="style.css">
  </head>
  <body>
    <div id="toolbar">
      <button value='coloured'>Colour</button>
      <button value='enlarged'>Enlarge</button>
      <button value='hidden'>Hide</button>
    </div>
    <div id="editor">
      <form name="editorForm">
        <label>Feelin': <input name="newText" placeholder="Hot"></label>
        <button id="setTextButton">Set</button>
      </form>
    </div>
    <div id="container">
      <div id="test">
        This is some text.
      </div>
    </div>
    <script src="script.js"></script>
  </body>
</html>

CSS (style.css):

html {
  box-sizing: border-box;
}
*, *:before, *:after {
  box-sizing: inherit;
}
#container {
  display: flex;
  align-items: center;
  justify-content: center;
  width: 100%;
  height: 100%;
}
#test {
  width: 200px;
  height: 100px;
  border: 1px solid black;
  padding: 1em;
}
.coloured {
  color: white;
  background: black;
}
.enlarged {
  font-size: 200%;
}
.hidden {
  display: none;
}

Javascript (script.js):

// Call 'onDocumentReady' when page is loaded
console.log('Script running');
if (document.readyState != 'loading'){
  onDocumentReady();
} else {
  document.addEventListener('DOMContentLoaded', onDocumentReady);
}

// Page is loaded! Now event can be wired-up
function onDocumentReady() {
  console.log('Document ready.');

  var el = document.getElementById('toolbar');
  // Listen to any click that happens on the toolbar
  // or its child elements (ie the BUTTONs)
  el.addEventListener('click', onButtonClicked);

  // Listen to 'click' on element with id 'setTextButton'
  document.getElementById('setTextButton').addEventListener('click', onSetTextClicked);
}

// Called according to 'click' event
function onButtonClicked(e) {
  console.log('onButtonClicked.');

  // Ignore clicks on the toolbar
  if (e.target.id == 'toolbar') return;

  // Get a reference to the element we want to manipulate
  // Here, we're using the id 'test'
  var el = document.getElementById('test');

  // Since each button has an assigned value, representing
  // which class to toggle, it's pretty easy:
  el.classList.toggle(e.target.value);
}

// 'Set' clicked
function onSetTextClicked(e) {
  console.log('onSetTextClicked.');
  e.preventDefault();

  // Get reference to element to change by id
  var el = document.getElementById('test');

  // Get entered text from INPUT
  var text = document.querySelector('input[name="newText"]').value;

  // Set text
  el.innerText = "Totally feelin' " + text;
}

Live demo of this on Glitch