body {
  font-family: sans-serif;
  text-align: center;
  margin: 30px;
}

h1 { margin-bottom: 20px; }

.controls {
  margin-bottom: 20px;
}

#display {
  font-size: 72px;
  font-weight: bold;
  margin: 20px 0;
  height: 100px;
  display: flex;
  align-items: center;
  justify-content: center;
}

.grid {
  display: grid;
  grid-template-columns: repeat(4, 1fr); /* 3 rows of 4 */
  gap: 10px;
  margin-top: 20px;
  max-width: 600px;
  margin-left: auto;
  margin-right: auto;
}

button, input[type=number] {
  font-size: 16px;
  padding: 8px 12px;
}

.interval-btn {
  padding: 10px;
  border: 1px solid #ccc;
  border-radius: 6px;
  background: #f9f9f9;
  cursor: pointer;
  font-size: 20px;
}

.interval-btn.active {
  background: #4CAF50;
  color: white;
}