wagtail/client/scss/layouts/_login.scss

@mixin login-fullscreen-background() {
  @at-root {
    :root {
      --w-login-fullscreen-background: radial-gradient(
        100% 300.28% at 100% 0%,
        theme('colors.primary.DEFAULT') 0%,
        theme('colors.primary.DEFAULT') 32.94%,
        theme('colors.primary.DEFAULT') 49.9%,
        theme('colors.primary.DEFAULT') 50.2%,
        theme('colors.secondary.DEFAULT') 100%
      );
    }
  }

  // support older browsers that do not support radial gradient 'at' usage
  background: theme('colors.surface-menus');
  /* stylelint-disable-next-line declaration-block-no-duplicate-properties */
  background: var(
    --w-login-fullscreen-background,
    theme('colors.primary.DEFAULT')
  );
}

.login {
  @include login-fullscreen-background;

  color: theme('colors.text-label');

  .wrapper {
    align-items: center;
    display: flex;
    flex-direction: column;
    height: auto;
    justify-content: center;
    padding: min(theme('spacing.4'), 5vh) min(theme('spacing.4'), 5vw);
    min-height: 100vh;
  }

  .messages {
    background: none;
    margin-bottom: theme('spacing.8');

    ul {
      @include unlist();

      li {
        border-radius: theme('borderRadius.sm');
        margin-bottom: theme('spacing.2');
      }
    }
  }

  .content-wrapper {
    // border style needed in forced color mode
    border: 3px solid transparent;
    height: auto;
    float: none;
    padding: theme('spacing.10') theme('spacing.12');
    background: theme('colors.surface-page');
    box-shadow: theme('boxShadow.md');
    border-radius: theme('borderRadius.md');
    margin-bottom: 15vh; // should appear slightly above centre
    max-width: calc(theme('spacing.32') * 3.15);

    @include media-breakpoint-up(sm) {
      max-width: calc(theme('spacing.32') * 3.5);
      padding: theme('spacing.12') theme('spacing.14');
    }
  }

  h1 {
    @apply w-h1;
    margin-top: 0;
    margin-bottom: theme('spacing.8');
    text-align: center;
  }

  .button,
  .w-field__label {
    width: 100%;
    font-size: theme('fontSize.16');
    font-weight: theme('fontWeight.bold');
  }

  .w-field__input {
    padding: 0; // reset padding for field not used inside the admin interface
  }

  .w-field__wrapper {
    margin-bottom: theme('spacing.6');
  }

  .skiplink {
    width: auto;
  }

  .reset-password {
    display: block;
    // bring in reset password link tight to the form
    margin: calc(-1 * theme('spacing.4')) 0 theme('spacing.8');
  }

  .remember-me {
    margin: theme('spacing.4') 0;
  }

  .form-actions {
    // no need for margin bottom due to padding on content-wrapper
    margin-top: theme('spacing.8');
    text-align: center;

    .button-longrunning {
      &-active {
        justify-content: center;
      }
    }
  }

  .login-form .w-field__errors {
    // for the login form, we rely on the non-field error only.

    display: none;
  }

  .login-logo {
    margin: theme('spacing.8') auto 0;
    width: theme('spacing.8');
  }

  @media (forced-colors: active) and (prefers-color-scheme: dark) {
    .login-logo-img {
      filter: invert(100%);
    }
  }
}