gecko/mobile/themes/core/notification.css

notification, notification[type="warning"] {
  padding: 0;
  color: #fff;
  background: #5768c3;
  border-top: 1px solid #4756aa;
  border-bottom: 1px solid #4756aa;
}

notification .messageCloseButton {
  list-style-image: url("chrome://browser/skin/images/close-default-hdpi.png");
  -moz-image-region: -moz-initial;
  -moz-box-align: start;
}

notification .messageCloseButton:hover:active {
  list-style-image: url("chrome://browser/skin/images/close-active-hdpi.png");
}

notification .outset {
  border-color: #4756aa !important;
}

notification .buttons {
  padding: 8px; /* core spacing */
}