body {
  margin: 0;
  display: flex;
  align-items: center;
  justify-content: center;
  height: 100vh;
  background-color: #1c1c1e;
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, sans-serif;
}

#calculator {
  display: grid;
  grid-template-columns: repeat(4, minmax(60px, 1fr));
  grid-auto-rows: 70px;
  gap: 12px;
  border-radius: 20px;
  padding: 20px;
  background-color: #1c1c1e;
  box-shadow: 0 0 20px rgba(0, 0, 0, 0.5);
  max-width: 320px;
  width: 100%;
}

#display {
  background-color: #2c2c2e;
  color: #ffffff;
  font-size: 2rem;
  text-align: right;
  padding: 20px;
  border-radius: 10px;
  grid-column: span 4;
  display: flex;
  align-items: center;
  justify-content: flex-end;
  word-break: break-word;
}

.btn {
  color: #ffffff;
  border: none;
  border-radius: 50%;
  font-size: 1.5rem;
  cursor: pointer;
  transition: filter 0.2s, transform 0.1s;
  box-shadow: 0 2px 4px rgba(0, 0, 0, 0.3);
}

.btn:active {
  transform: scale(0.95);
}

.btn:hover {
  filter: brightness(1.2);
}

.number {
  background-color: #505050;
}

.operator,
.equal {
  background-color: #e63946;
}

.function {
  background-color: #a6a6a6;
}

#zero {
  grid-column: span 2;
}

@media (max-width: 400px) {
  #calculator {
    grid-template-columns: repeat(4, 1fr);
    grid-auto-rows: 60px;
  }

  #display {
    font-size: 1.5rem;
    padding: 15px;
  }
}