wagtail/client/scss/components/_modals.scss

$zindex-modal-background: 500;

.fade {
  @include transition(opacity 0.15s linear);
  opacity: 0;

  &.in {
    opacity: 1;
  }
}

// Kill the scroll on the body
.modal-open {
  overflow: hidden;

  .content-wrapper {
    transform: none;
  }
}

// Container that the modal scrolls within
.modal {
  box-sizing: border-box;
  display: none;
  overflow: auto;
  overflow-y: scroll;
  position: fixed;
  top: 0;
  inset-inline-end: 0;
  bottom: 0;
  inset-inline-start: 0;
  z-index: $zindex-modal-background;
}

// Shell div to position the modal with bottom padding
.modal-dialog {
  box-sizing: border-box;
  margin-inline-start: auto;
  margin-inline-end: auto;
  padding: 0;
  z-index: ($zindex-modal-background + 10);
  height: 90%;
  width: 85%;

  &:before {
    content: '';
    display: inline-block;
    height: 100%;
    vertical-align: middle;
    margin-inline-end: -0.25em;
  }
}

// Actual modal
.modal-content {
  box-sizing: border-box;
  border-radius: 3px;
  width: 98.7%;
  position: relative;
  background-color: $color-white;
  margin-top: 2em;
  padding-bottom: 3em;
  display: inline-block;
  vertical-align: middle;
  overflow: hidden;
}

// Modal background
.modal-backdrop {
  position: fixed;
  top: 0;
  inset-inline-end: 0;
  bottom: 0;
  inset-inline-start: 0;
  z-index: ($zindex-modal-background - 10);
  background-color: $color-black;

  // Fade for backdrop
  &.fade {
    opacity: 0;
  }

  &.in {
    opacity: 0.5;
  }
}

.modal .close.button {
  position: absolute;
  display: inline-flex;
  justify-content: center;
  align-items: center;
  background-color: transparent;
  padding: 0;
  top: theme('spacing.3');
  inset-inline-end: theme('spacing.3');
  width: theme('spacing.8');
  height: theme('spacing.8');
  color: theme('colors.white.DEFAULT');
  z-index: 1;

  &:hover {
    background-color: theme('colors.black.DEFAULT');
    color: theme('colors.grey.50');
  }
}

// Where all modal content resides
.modal-body {
  position: relative;
  padding-bottom: 2em;

  .w-header {
    color: theme('colors.white.DEFAULT');
    background-color: theme('colors.primary.DEFAULT');

    > .row {
      // reset inline padding added to w-header for sidebar offset
      padding-inline-start: theme('spacing.16');
      // ensure title can never overlap the close button
      padding-inline-end: theme('spacing.16');
    }

    .w-header__title {
      color: inherit;
      font-weight: 700;
      font-size: 1.125rem;
      line-height: 130%;
    }
  }
}

@include media-breakpoint-up(sm) {
  .modal-dialog {
    margin-inline-end: theme('spacing.20');
    margin-inline-start: theme('spacing.20');
    padding: 0 0 2em 0;
    width: auto;
  }

  .modal .close.button {
    width: theme('spacing.12');
    height: theme('spacing.12');
  }
}

@include media-breakpoint-up(xl) {
  .modal-dialog {
    margin-inline-end: auto;
    margin-inline-start: auto;
    max-width: 100em;
    padding: 0 0 2em;
  }
}