<div class="items">
  <div class="item red"></div>
  <div class="item green"></div>
  <div class="item blue"></div>
  <div class="item yellow"></div>
</div>

html,
body {
  min-height: 100%;
  margin: 0;
  padding: 0;
}

.items {
  display: flex;
}

.red {
  background: red;
}

.green {
  background: green;
}

.blue {
  background: blue;
}

.yellow {
  background: yellow;
}

.item {
  width: 40px;
  height: 40px;
  border: 1px solid #fff;
  margin-left: 10px;
  margin-top: 10px;
  font-size: 0;
  cursor: pointer;
}

document.querySelectorAll(".item").forEach(function (el) {
  el.addEventListener("click", function (event) {
    document.body.style.backgroundColor = event.target.className.slice(4);
  });
});