* {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
  font-family: "Poppins", sans-serif;
}

body {
  background: linear-gradient(135deg, #1d1d1d, #3b3b3b);
  display: flex;
  justify-content: center;
  align-items: center;
  min-height: 100vh;
  text-align: center;
  padding: 20px;
}

.container {
  width: 100%;
  max-width: 500px;
  color: #fff;
}

h1 {
  font-size: 2.5rem;
  margin-bottom: 10px;
  color: #f1c40f;
  text-shadow: 0px 3px 6px rgba(0,0,0,0.5);
}

.tagline {
  color: #ccc;
  margin-bottom: 25px;
  font-size: 1.2rem;
}

.calculator {
  background: #222;
  padding: 25px;
  border-radius: 20px;
  width: 100%;
  box-shadow: 0px 10px 25px rgba(0,0,0,0.6);
}

.display input {
  width: 100%;
  height: 80px;
  border: none;
  outline: none;
  background: #111;
  color: #fff;
  text-align: right;
  font-size: 2.5rem;
  padding: 15px;
  border-radius: 12px;
  margin-bottom: 20px;
}

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

.btn {
  background: #333;
  color: #fff;
  border: none;
  padding: 20px;
  font-size: 1.5rem;
  border-radius: 12px;
  cursor: pointer;
  transition: all 0.2s ease;
}

.btn:hover {
  background: #555;
  transform: scale(1.05);
}

.clear {
  background: #e74c3c;
}

.clear:hover {
  background: #c0392b;
}

.equal {
  background: #27ae60;
  grid-row: span 2;
}

.equal:hover {
  background: #1e8449;
}

.zero {
  grid-column: span 2;
}

.footer {
  margin-top: 20px;
  font-size: 1rem;
  color: #aaa;
}

@media (max-width: 500px) {
  h1 {
    font-size: 2rem;
  }

  .tagline {
    font-size: 1rem;
  }

  .btn {
    padding: 18px;
    font-size: 1.3rem;
  }

  .display input {
    height: 70px;
    font-size: 2rem;
  }
}