wekan/client/components/notifications/notifications.css

#notifications {
  position: relative;
}
#notifications .notifications-drawer-toggle {
  display: block;
  line-height: 28px;
  color: #f2f2f2;
  margin: 0 10px;
  width: 28px;
  height: 28px;
  text-align: center;
  border: 0;
  padding: 0;
}
#notifications .notifications-drawer-toggle.alert {
  background-color: #eb4646;
}