wagtail/client/scss/elements/_elements.scss

html {
  background: $color-grey-4;
  height: 100%;
}

body {
  overflow-x: hidden;
  position: relative;

  &:after {
    content: '';
    position: fixed;
    transition: visibility 0s linear 0s, opacity 0.2s ease-out;
    background: rgba(255, 255, 255, 0.5);
    width: 100%;
    height: 100%;
    top: 0;
    // Remove once we drop support for Safari 13.
    // stylelint-disable-next-line property-disallowed-list
    left: 0;
    inset-inline-start: 0;
    z-index: 5;
    opacity: 0;
    visibility: hidden;
  }
}

hr {
  border: 1px solid $color-grey-4;
  border-width: 1px 0 0;
  margin: 1.5em 0;
}

// general image style
img {
  max-width: 100%;
  height: auto;
}