vikunja-frontend/src/styles/theme/notification.scss

.notification {
  border: $thickness solid $border;

  @each $name, $pair in $colors {
    $color: nth($pair, 1);

    &.is-#{$name} {
      border-color: darken($color, 5);
    }
  }
}

.notifications {
  left: 0.5em !important;
  bottom: 1em !important;

  .notification-wrapper .notification {
    -webkit-border-radius: 0;
    -moz-border-radius: 0;
    border-radius: 0;
    border-top-width: 0;
    border-right-width: 0;
    border-bottom-width: 0;
    border-left-width: 0.4em;
  }
}

.message {
  .message-body {
    border: $thickness solid;
  }
}