KNACSS/sass/components/_radio.scss

/* ----------------------------- */
/* ==Rdio                    */
/* ----------------------------- */
/* use .radio class on input type=radio */
/* recommanded HTML : <input type="radio" class="radio" name="radio" id="r1"><label for="r1">Click here</label> */

.radio {
  @extend .visually-hidden;

  & ~ label {
    display: flex;
    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: 50%;
      box-shadow:  0 0 0 2px $color-checkbox inset;
      transition: .25s background;
    }
  }
  &:checked ~ label:before {
    background-image: radial-gradient(circle at center, #fff 30%, transparent 38%);
    background-color: $color-checkbox;
    background-position: center;
    background-repeat: no-repeat;
    pointer-events: none;
  }
}