选中时无法更改字体棒的图标颜色-复选框

<label class="checkbox-container">
  <input type="checkbox">
  <span class="checkmark"></span>
  Option 1
</label>

.checkbox-container {
  display: inline-block;
  position: relative;
  padding-left: 25px;
  margin-bottom: 12px;
  cursor: pointer;
  font-size: 18px;
}

.checkbox-container input {
  position: absolute;
  opacity: 0;
  cursor: pointer;
}

.checkmark {
  position: absolute;
  top: 0;
  left: 0;
  height: 20px;
  width: 20px;
  background-color: #eee;
}

.checkbox-container input:checked ~ .checkmark {
  background-color: #2196F3;
}

.checkmark:after {
  content: "";
  position: absolute;
  display: none;
}

.checkbox-container input:checked ~ .checkmark:after {
  display: block;
}

.checkbox-container .checkmark:after {
  left: 7px;
  top: 3px;
  width: 5px;
  height: 10px;
  border: solid white;
  border-width: 0 3px 3px 0;
  transform: rotate(45deg);
}

var checkbox = document.querySelector('input[type="checkbox"]');
var checkmark = document.querySelector('.checkmark');

checkbox.addEventListener('change', function() {
  if (this.checked) {
    checkmark.style.backgroundColor = '#2196F3';
  } else {
    checkmark.style.backgroundColor = '#eee';
  }
});