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

/* Container to center everything */
.container {
  text-align: center;
  padding: 20px;
  margin-bottom: 2.3rem;
}

/* Heading style */
.heading {
  color: #fff;
  font-size: 36px;
  margin-bottom: 25px;
}

.calculator {
  width: 320px;
  padding: 10px 8px;
  background: #000;
  border-radius: 20px;
  overflow: hidden;
   border: 2px solid #333; /* added subtle border */
  box-shadow: 0 0 30px rgba(255,255,255,0.1), 
              0 0 10px rgba(255,255,255,0.2); /* added border shadow */
}

.display {
  color: #fff;
  text-align: right;
  padding: 30px 20px;
  font-size: 48px;
  word-wrap: break-word;
}

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

.btn {
  font-size: 28px;
  margin: 5px;
  padding: 15px 0px;
  border: black;
  border-radius: 10px;
  background: #333;
  color: #fff;
  cursor: pointer;
}

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

.function {
  font-size: 25px;
  background: #5c5b5b;
  color: #fff;
}

.operator {
  background: #ff9500;
  color: #fff;
}

.equal {
  font-size: 28px;
  margin: 5px;
  padding: 15px;
  border: black;
  border-radius: 10px;
  background: #333;
  color: #fff;
  cursor: pointer;
}

.equal:active {
  filter: brightness(1.2);
}

.percentage {
  font-size: 28px;
  margin: 5px;
  padding: 15px;
  border: black;
  border-radius: 10px;
  background: #333;
  color: #fff;
  cursor: pointer;
}

.percentage:active {
  filter: brightness(1.2);
}

/* Make responsive for small screens */
@media (max-width: 480px) {
  .container {
    margin-bottom: 2.5rem;
  }

  .calculator {
    width: 95%;
    padding: 8px 5px;
  }

  .display {
    font-size: 40px;
    padding: 20px 10px;
  }

  .btn, .equal, .percentage {
    font-size: 30px;
    margin: 4px;
    padding: 15px 5px;
  }

  .function {
    font-size: 30px;
  }

  .heading {
    font-size: 32px;
    margin-bottom: 25px;
  }
}