KNACSS/sass/components/checkbox.scss

/* ----------------------------- */
/* ==Checkbox                    */
/* ----------------------------- */
/* use .checkbox class on input type=checkbox */
/* recommanded HTML : <input type="checkbox" class="checkbox" id="c1"><label for="c1">click here</label> */

.checkbox {
  @extend .visually-hidden;

  & ~ label {
    cursor: pointer;

    &:before {
      content: "";
      display: inline-block;
      vertical-align: baseline;
      transform: translateY(.2rem);
      margin-right: .5rem;
      width: 1.1em; height: 1.1em;
      border: 0;
      background: #fff;
      border-radius: $border-radius;
      box-shadow:  0 0 0 2px $color-forms inset;
      transition: .25s background-color;
    }
  }
  &:checked ~ label:before {
    background-image: url(data:image/svg+xml;base64,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);
    background-color: $color-forms;
    background-size: 60% 60%;
    background-position: center;
    background-repeat: no-repeat;
    pointer-events: none;
  }
}