body {
  font-family: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif;
  background: linear-gradient(135deg, #dbeafe, #f0f9ff);
  margin: 0;
  padding: 0;
  display: flex;
  flex-direction: column;
  min-height: 100vh;
}

.container {
  text-align: center;
  padding: 30px 20px;
  background-color: white;
  margin: 40px auto;
  max-width: 700px;
  border-radius: 20px;
  box-shadow: 0 10px 30px rgba(0, 0, 0, 0.1);
}

h1 {
  color: #1e3a8a;
  margin-bottom: 30px;
}

.led-grid {
  display: grid;
  grid-template-columns: repeat(4, 1fr);
  gap: 15px;
  margin-bottom: 30px;
}

.led {
  width: 70px;
  height: 70px;
  border-radius: 50%;
  background: radial-gradient(circle at 30% 30%, #fff, gray);
  box-shadow: 0 0 10px gray;
  cursor: pointer;
  transition: transform 0.2s, box-shadow 0.3s;
}

.led.active {
  transform: scale(1.1);
  border: 3px solid #3b82f6;
}

.controls {
  display: flex;
  flex-direction: column;
  align-items: center;
}

.volt-display {
  font-size: 20px;
  font-weight: bold;
  color: #334155;
  margin: 10px 0;
}

input[type="range"] {
  width: 80%;
  margin-top: 10px;
  accent-color: #3b82f6;
}

footer {
  text-align: center;
  padding: 20px;
  color: #64748b;
  font-size: 14px;
  margin-top: auto;
}