wagtail/client/scss/layouts/_404.scss

.page404__bg {
  position: fixed;
  top: 0;
  inset-inline-start: 0;
  width: 100vw;
  height: 100vh;
  background-color: theme('colors.secondary.400');
  font-family: theme('fontFamily.sans');
  color: theme('colors.white.DEFAULT');
}

.page404__logo {
  width: 400px;
  height: 500px;

  // Media for Windows High Contrast mode
  @media (forced-colors: active) {
    background-color: LinkText;
  }
}

.page404__text-container {
  width: 600px;

  height: 500px;
  text-align: center;
}

.page404__header {
  font-size: 6.8em;
  margin-bottom: 0.2em;
  color: inherit;
}

.page404__text {
  font-size: 2.25em;
  line-height: 1.25em;
  color: inherit;
}

a.button.page404__button {
  // more specific to override standard button styles
  font-size: 1.5em;
  line-height: 2em;
  height: 2.5em;
  padding: 0 0.5em;
  background-color: theme('colors.secondary.400');
  border: 4px solid theme('colors.secondary.DEFAULT');
  color: inherit;

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

// MOBILE CHANGES:
@include media-breakpoint-down(xs) {
  .page404__text-container {
    width: 400px;
  }

  .page404__header {
    font-size: 5em;
  }
}