ionic-framework/scss/ionic/_checkbox.scss

.checkbox {
  position: relative;
  display: inline-block;
  padding: ($checkbox-height / 4) ($checkbox-width / 4);
  cursor: pointer;

  input {
    display: none;
  }

  /* what the checkbox looks like when its not checked */
  .handle {
    width: $checkbox-width;
    height: $checkbox-height;
    border: $checkbox-border-width solid $checkbox-off-border-color;
    border-radius: $checkbox-border-radius;
    background: $checkbox-off-bg-color;
    transition: background-color .1s ease-in-out;
  }

  /* the checkmark within the box */
  .handle:after {
    position: absolute;
    top: 37%;
    left: $checkbox-width / 2;
    width: $checkbox-width / 2;
    height: $checkbox-width / 4;
    border: $checkbox-check-width solid $checkbox-check-color;
    border-top: none;
    border-right: none;
    content: '';
    opacity: 0;
    transition: opacity .05s ease-in-out;

    -webkit-transform: rotate(-45deg);
    transform: rotate(-45deg);
  }

  /* what it looks like when it is checked */
  input:checked + .handle {
    background: $checkbox-on-bg-color;

    &:after {
      opacity: 1;
    }
  }

}