mdb-ui-kit/scss/includes/_radios.scss

@mixin radio-color($color, $opacity){
  & ~ .check,
  & ~ .circle {
    opacity: $opacity;
  }

  & ~ .check {
    background-color: $color;
  }

  & ~ .circle {
    border-color: $color;
  }
}

.radio {
  label {
    cursor: pointer;
    padding-left: 45px;
    position: relative;
    color: $mdb-radio-label-color;

    span {
      display: block;
      position: absolute;
      left: 10px;
      top: 2px;
      transition-duration: 0.2s;
    }
    .circle {
      border: 2px solid $mdb-radio-color-off;
      height: 15px;
      width: 15px;
      border-radius: 100%;
    }
    .check {
      height: 15px;
      width: 15px;
      border-radius: 100%;
      background-color: $mdb-radio-color-on;
      transform: scale3d(0, 0, 0);
    }
    .check:after {
      display: block;
      position: absolute;
      content: "";
      background-color: $mdb-text-color-primary;
      left: -18px;
      top: -18px;
      height: 50px;
      width: 50px;
      border-radius: 100%;
      z-index: 1;
      opacity: 0;
      margin: 0;
      transform: scale3d(1.5, 1.5, 1);
    }
    input[type=radio]:not(:checked) ~ .check:after {
      animation: rippleOff 500ms;
    }
    input[type=radio]:checked ~ .check:after {
      animation: rippleOn 500ms;
    }

  }

  input[type=radio] {
    opacity: 0;
    height: 0;
    width: 0;
    overflow: hidden;

    &:checked {
      @include radio-color($mdb-radio-color-on, 1);
    }
    &:checked ~ .check {
      transform: scale3d(0.55, 0.55, 1);
    }
  }

  input[type=radio][disabled] {

    // light theme spec: Disabled: #000000, Opacity  26%
    @include radio-color($black, 0.26);

    // dark theme spec: Disabled: #FFFFFF, Opacity  30%
    .theme-dark & {
      @include radio-color($white, 0.30);
    }
  }
}

@keyframes rippleOn {
  0% {
    opacity: 0;
  }
  50% {
    opacity: 0.2;
  }
  100% {
    opacity: 0;
  }
}

@keyframes rippleOff {
  0% {
    opacity: 0;
  }
  50% {
    opacity: 0.2;
  }
  100% {
    opacity: 0;
  }
}