Create custom checkbox

custom-checkbox.liquid

<label for="custom-checkbox" class="custom-checkbox">
  <input id="custom-checkbox" type="checkbox" name="custom_checkbox">
  <span>Custom checkbox</span>
</label>

<style>
  .custom-checkbox > input[type=checkbox] {
    opacity: 0;
    cursor: pointer;
    position: absolute;
  }

  .custom-checkbox > span {
    display: block;
    position: relative;
    padding-left: 1.5em;
    line-height: 1.3;
  }

  .custom-checkbox > span:before,
  .custom-checkbox > span:after {
    content: '';
    position: absolute;
  }

  .custom-checkbox > span:before {
    left: 0;
    top: 0;
    width: 1.3em;
    height: 1.3em;
    border: 1px solid currentColor;
    border-radius: 3px;
  }

  .custom-checkbox > span:after {
    opacity: 0;
    width: 0.7em;
    height: 0.4em;
    border-left: 2px solid #fff;
    border-bottom: 2px solid #fff;
    left: 0.3em;
    top: 0.3em;
    transform: rotate(-45deg);
  }

  .custom-checkbox > input[type=checkbox]:checked + span:before {
    background-color: currentColor;
  }

  .custom-checkbox > input[type=checkbox]:checked + span:after {
    opacity: 1;
  }
</style>