metamask-extension/ui/app/css/itcss/components/alert.scss

.global-alert {
  position: relative;
  width: 100%;
  background-color: #33a4e7;

  .msg {
    width: 100%;
    display: block;
    color: white;
    font-size: 12px;
    text-align: center;
  }
}

.global-alert.hidden {
  animation: alertHidden 0.5s ease forwards;
}

.global-alert.visible {
  animation: alert 0.5s ease forwards;
}

/* Animation */
@keyframes alert {
  0% {
    opacity: 0;
    top: -50px;
    padding: 0;
    line-height: 12px;
  }

  50% {
    opacity: 1;
  }

  100% {
    top: 0;
    padding: 8px;
    line-height: 12px;
  }
}

@keyframes alertHidden {
  0% {
    top: 0;
    opacity: 1;
    padding: 8px;
    line-height: 12px;
  }

  100% {
    opacity: 0;
    top: -50px;
    padding: 0;
    line-height: 0;
  }
}