mdb-ui-kit/scss/_checkboxes.scss

// http://www.google.com/design/spec/components/selection-controls.html#selection-controls-checkbox
.checkbox label,
label.checkbox-inline {
  padding-left: $bmd-checkbox-size + $bmd-checkbox-label-padding; // absolutely positioned so add the radio size

  .checkbox-decorator {
    position: absolute;
    left: 0;
    padding: .7em;
    margin: -.7em;
    line-height: .7;
    vertical-align: middle;
    cursor: pointer;
    border-radius: 100%;

    .check {
      position: relative;
      z-index: 1;
      display: inline-block;
      width: $bmd-checkbox-size;
      height: $bmd-checkbox-size;
      overflow: hidden;
      border: $bmd-checkbox-border-size solid $bmd-checkbox-border-color;
      border-radius: $border-radius;

      // checkbox outline
      &::before {
        position: absolute;
        display: block;
        width: 0;
        height: 0;
        margin-top: -4px;
        margin-left: 6px;
        // we need a solid color here to avoid glitches in the animation
        // it will not look great on non white backgrounds, but this is the
        // most common case and it's better support it
        color: lighten(rgba($gray, 1),  (1 - $gray-alpha) * 100);
        content: "";
        box-shadow: 0 0 0 0, 0 0 0 0, 0 0 0 0, 0 0 0 0, 0 0 0 0, 0 0 0 0,
          0 0 0 0 inset;
        transform: rotate(45deg);
        animation: checkbox-off;

        .is-focused & {
          // Prevent checkbox animation and ripple effect on page load
          animation: checkbox-off $bmd-checkbox-animation-check forwards;
        }
      }
    }
  }

  input[type=checkbox] {
    // Hide native checkbox
    position: absolute;
    left: 0;
    z-index: -1;
    width: 0;
    height: 0;
    margin: 0;
    overflow: hidden;
    pointer-events: none;
    opacity: 0;

    &:focus + .checkbox-decorator .check::after {
      opacity: 0.2;
    }

    &:checked {
      // FIXME: once working - combine further to reduce code
      + .checkbox-decorator .check {
        color: $bmd-checkbox-checked-color;
        border-color: $bmd-checkbox-checked-color;
      }

      + .checkbox-decorator .check::before {
        color: $bmd-checkbox-checked-color;
        box-shadow: 0 0 0 10px, 10px -10px 0 10px, 32px 0 0 20px, 032px 0 20px,
          -5px 5px 0 10px, 20px -12px 0 11px;
        animation: checkbox-on $bmd-checkbox-animation-check forwards;
      }
    }

    &[disabled],
    fieldset[disabled] & {
      + .checkbox-decorator .check::after,
      .check::after {
        background-color: $gray-dark;
        transform: rotate(-45deg);
      }
      + .checkbox-decorator .check,
      .check {
        border-color: $bmd-checkbox-border-color-disabled;
      }
      // No ripple on disabled checkboxes
      + .checkbox-decorator .ripple-container {
        display: none;
      }
    }
  }
}

@keyframes checkbox-on {
  0% {
    box-shadow: 0 0 0 10px, 10px -10px 0 10px, 32px 0 0 20px, 0 32px 0 20px,
      -5px 5px 0 10px, 15px 2px 0 11px;
  }
  50% {
    box-shadow: 0 0 0 10px, 10px -10px 0 10px, 32px 0 0 20px, 0 32px 0 20px,
      -5px 5px 0 10px, 20px 2px 0 11px;
  }
  100% {
    box-shadow: 0 0 0 10px, 10px -10px 0 10px, 32px 0 0 20px, 0 32px 0 20px,
      -5px 5px 0 10px, 20px -12px 0 11px;
  }
}

@keyframes checkbox-off {
  0% {
    box-shadow: 0 0 0 10px, 10px -10px 0 10px, 32px 0 0 20px, 0 32px 0 20px,
      -5px 5px 0 10px, 20px -12px 0 11px, 0 0 0 0 inset;
  }
  25% {
    box-shadow: 0 0 0 10px, 10px -10px 0 10px, 32px 0 0 20px, 0 32px 0 20px,
      -5px 5px 0 10px, 20px -12px 0 11px, 0 0 0 0 inset;
  }
  50% {
    width: 0;
    height: 0;
    margin-top: -4px;
    margin-left: 6px;
    box-shadow: 0 0 0 10px, 10px -10px 0 10px, 32px 0 0 20px, 0 32px 0 20px,
      -5px 5px 0 10px, 15px 2px 0 11px, 0 0 0 0 inset;
    transform: rotate(45deg);
  }
  51% {
    width: 20px;
    height: 20px;
    margin-top: -2px;
    margin-left: -2px;
    box-shadow: 0 0 0 0, 0 0 0 0, 0 0 0 0, 0 0 0 0, 0 0 0 0, 0 0 0 0,
      0 0 0 10px inset;
    transform: rotate(0deg);
  }
  100% {
    width: 20px;
    height: 20px;
    margin-top: -2px;
    margin-left: -2px;
    box-shadow: 0 0 0 0, 0 0 0 0, 0 0 0 0, 0 0 0 0, 0 0 0 0, 0 0 0 0,
      0 0 0 0 inset;
    transform: rotate(0deg);
  }
}