ant-design/style/components/dialog/Mask.less

.@{dialog-prefix-cls} {

  &-wrap-hidden > &-mask {
    display: none;
  }

  &-mask {
    position: fixed;
    top: 0;
    right: 0;
    left: 0;
    bottom: 0;
    background-color: #373737;
    background-color: rgba(55, 55, 55, 0.6);
    height: 100%;
    z-index: 1000;
    filter: alpha(opacity=50);
  }

  &-wrap-hidden > &-mask&-fade-enter,
  &-wrap-hidden > &-mask&-fade-leave {
    display: block;
  }

  .fade-effect() {
    animation-duration: 0.3s;
    animation-fill-mode: both;
    animation-timing-function: @ease-in;
  }

  &-fade-enter {
    opacity: 0;
    .fade-effect();
    animation-play-state: paused;
  }

  &-fade-leave {
    .fade-effect();
    animation-play-state: paused;
  }

  &-fade-enter&-fade-enter-active {
    animation-name: fadeIn;
    animation-play-state: running;
  }

  &-fade-leave&-fade-leave-active {
    animation-name: fadeOut;
    animation-play-state: running;
  }
}