edx-platform/lms/static/sass/shared/_modal.scss

#lean_overlay {
    background: transparent;
    @include background-image(radial-gradient(50% 30%, circle cover, rgba(0,0,0, 0.3), rgba(0,0,0, 0.8)));
    display: none;
    height:100%;
    left: 0px;
    position: fixed;
    top: 0px;
    width:100%;
    z-index:100;
}

.modal {
  background: rgba(0,0,0, 0.6);
  border: 1px solid rgba(0, 0, 0, 0.9);
  border-radius: 0px;
  box-shadow: 0 15px 80px 15px rgba(0,0,0, 0.5);
  color: #fff;
  display: none;
  left: 50%;
  padding: 8px;
  position: absolute;
  width: grid-width(5);
  z-index: 12;

  &.video-modal {
    left: 50%;
    padding: 10px;
    width: 582px;

    .inner-wrapper {
      background: #000;
      box-shadow: none;
      height: 315px;
      padding: 10px;
      width: 560px;
    }
  }

  &.home-page-video-modal {
    left: 50%;
    padding: 10px;
    width: 662px;

    .inner-wrapper {
      background: #000;
      box-shadow: none;
      height: 360px;
      padding: 10px;
      width: 640px;
    }
  }

  .inner-wrapper {
    background: $modal-bg-color;
    border-radius: 0px;
    border: 1px solid rgba(0, 0, 0, 0.9);
    box-shadow: inset 0 1px 0 0 rgba(255, 255, 255, 0.7);
    overflow: hidden;
    padding-left: 10px;
    padding-right: 10px;
    padding-bottom: 30px;
    position: relative;
    z-index: 2;

    header {
      margin-bottom: 30px;
      overflow: hidden;
      padding: 28px 20px 0px;
      position: relative;
      z-index: 2;

      &::before {
        @include background-image(radial-gradient(50% 50%, circle closest-side, rgba(255,255,255, 0.8) 0%, rgba(255,255,255, 0) 100%));
        content: "";
        display: block;
        height: 400px;
        left: 0px;
        margin: 0 auto;
        position: absolute;
        top: -140px;
        width: 100%;
        z-index: 1;
      }

      hr {
        @extend %faded-hr-divider-light;
        border: none;
        margin: 0px;
        position: relative;
        z-index: 2;

        &::after {
          @extend %faded-hr-divider;
          bottom: 0px;
          content: "";
          display: block;
          position: absolute;
          top: -1px;
        }
      }

      h2 {
        position: relative;
        text-align: center;
        text-shadow: 0 1px rgba(255,255,255, 0.4);
        z-index: 2;
      }
    }

    .modal-form-error {
      background: tint($red,90%);
      border: 1px solid rgb(143, 14, 14);
      color: rgb(143, 14, 14);
      display: none;
      margin-bottom: 20px;
      padding: 12px;
    }

    .notice {
      background: $yellow;
      border: 1px solid darken($yellow, 60%);
      color: darken($yellow, 60%);
      display: none;
      margin-bottom: 20px;
      padding: 12px;
    }

    .activation-message, .message {
      padding: 0 40px 10px;

      p {
        margin-bottom: 10px;
      }
    }

    form {
      margin-bottom: 12px;
      padding: 0px 40px;
      position: relative;
      z-index: 2;

      .input-group {
        @include clearfix;
        border-bottom: 1px solid rgb(210,210,210);
        box-shadow: 0 1px 0 0 rgba(255,255,255, 0.6);
        margin-bottom: 30px;
        padding-bottom: 10px;
      }

      label {
        color: $text-color;

        &.field-error {
          display: block;
          color: #8F0E0E;

          + input, + textarea {
            border: 1px solid #CA1111;
            color: #8F0E0E;
          }
        }
      }

      input[type="checkbox"] {
        margin-right: 5px;
      }

      textarea {
        background: rgb(255,255,255);
        display: block;
        height: 45px;
        margin-bottom: 20px;
        width: 100%;
      }

      input[type="email"],
      input[type="text"],
      input[type="password"] {
        background: rgb(255,255,255);
        display: block;
        height: 45px;
        margin-bottom: 20px;
        width: 100%;
      }

      label.remember-me,
      label.terms-of-service,
      label.honor-code {
        background: rgb(233,233,233);
        border: 1px solid rgb(200,200,200);
        border-radius: 3px;
        box-shadow: 0 1px 0 0 rgba(255,255,255, 0.6);
        display: block;
        margin-bottom: 20px;
        padding: 8px 10px;

        &:hover, &:focus {
          background: rgb(230,230,230);
        }

        &.field-error {
          border: 1px solid #CA1111;
        }

        a {
          font-family: $serif;
          font-style: italic;
        }
      }

      .citizenship, .gender, .date-of-birth {
        margin-bottom: 20px;
        float: left;
        width: flex-grid(4);

        label {
          display: block;
        }

        select {
          width: 100%;
          @include box-sizing(border-box);
          display: block;
        }
      }

      .citizenship, .gender {
        margin-right: flex-gutter();
      }

      .submit {
        padding-top: 10px;

        input[type="submit"] {
          display: block;
          height: 45px;
          margin: 0 auto;
          width: 100%;
        }
      }
    }

    .login-extra {
      position: relative;
      z-index: 2;

      p {
        color: $lighter-base-font-color;
        font-style: italic;
        text-align: center;

        span {
          color: $lighter-base-font-color;
          font-family: $serif;
          font-style: italic;
        }

        a {
          color: $lighter-base-font-color;
          font-family: $serif;
          font-style: italic;
          text-decoration: underline;

          &:hover, &:focus {
            color: $base-font-color;
          }
        }

        span + a {
          margin-left: 15px;
        }
      }
    }

    .close-modal {
      border-radius: 2px;
      cursor: pointer;
      @include inline-block;
      padding: 10px;
      position: absolute;
      right: 2px;
      top: 0px;
      z-index: 3;
      color: $lighter-base-font-color;
      font: normal 1.2rem/1.2rem $sans-serif;
      text-align: center;
      text-shadow: 0 1px rgba(255,255,255, 0.8);
      @include transition(all 0.15s ease-out 0s);

      &:hover, &:focus {
        color: $base-font-color;
        text-decoration: none;
      }
    }
  }

  #help_wrapper, .discussion-alert-wrapper {
    padding: 0 ($baseline*1.5) ($baseline*1.5) ($baseline*1.5);

    header {
      margin-bottom: $baseline;
      padding-right: 0;
      padding-left: 0;
    }


  }

  .discussion-alert-wrapper button {
    display: block;
    margin: 0 auto;
  }

  .tip {
    font-size: 12px;
    display: block;
    color: $dark-gray;
  }


}

.leanModal_box {
  @extend .modal;
}