如何创建混合实现的不确定复选框？

<input type="checkbox" id="checkbox1" value="value1" name="checkboxes">

input[type="checkbox"] {
  position: relative;
  width: 20px;
  height: 20px;
  -webkit-appearance: none;
  -moz-appearance: none;
  appearance: none;
  border: 2px solid #ccc;
  border-radius: 4px;
  outline: none;
  cursor: pointer;
}

input[type="checkbox"]::before {
  content: "";
  position: absolute;
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
  width: 10px;
  height: 10px;
  background-color: transparent;
  border: 2px solid #ccc;
  border-top: none;
  border-right: none;
  transform-origin: center;
  opacity: 0;
}

input[type="checkbox"]:checked::before {
  opacity: 1;
  background-color: #ccc;
}

const checkbox = document.getElementById("checkbox1");

checkbox.addEventListener("click", function() {
  if (checkbox.checked) {
    checkbox.indeterminate = false;
  } else {
    checkbox.indeterminate = true;
  }
});