body, html {
  margin: 0;
  padding: 0;
  font-family: Arial, sans-serif;
}
#loginPage {
  display: flex;
  justify-content: center;
  align-items: center;
  height: 100vh;
  background-color: #2e7d32;
}
#loginBox {
  background-color: #388e3c;
  padding: 30px;
  border-radius: 10px;
  color: white;
}
#systemPage {
  display: none;
  flex-direction: column;
  height: 100vh;
}
header {
  background: #2e7d32;
  color: white;
  padding: 10px;
  display: flex;
  justify-content: space-between;
}
nav {
  background: #1b5e20;
  display: flex;
}
nav button {
  flex: 1;
  padding: 10px;
  color: white;
  background: none;
  border: none;
  cursor: pointer;
}
nav button.active {
  background-color: #33691e;
}
main {
  flex-grow: 1;
  padding: 20px;
  background: #f0f0f0;
}
section {
  display: none;
}
section.active {
  display: block;
}