body {
  font-family: Arial, sans-serif;
  background-color: #000;
  color: #fff;
  margin: 0;
  padding: 0;
}
header {
  background-color: #a00;
  padding: 1rem;
  text-align: center;
}
nav {
  margin-top: 1rem;
}
nav button {
  margin: 0 0.5rem;
  padding: 0.5rem 1rem;
  border: none;
  background: #222;
  color: #fff;
  cursor: pointer;
}
main {
  padding: 1rem;
}
.tab {
  display: none;
}
.tab.active {
  display: block;
}
.online {
  color: #0f0;
}
.alerta {
  color: #ff0;
}
.offline {
  color: #f00;
}