body {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
  display: flex;
  flex-direction: column; 
  min-height: 100vh; 
  justify-content: center; 
  align-items: center; 
  background: url("https://images.pexels.com/photos/3729557/pexels-photo-3729557.jpeg?auto=compress&cs=tinysrgb&w=800"), linear-gradient(135deg, #1e1e1e, #3a3a3a);
  background-repeat: no-repeat;
  background-size: cover;
  background-position: center;
}

header {
  background-color: rgba(0, 0, 0, 0.6); 
  color: white; 
  padding: 10px;
  text-align: center;
  width: 100%; 
}

header h1 {
  font-weight: bold;
  margin: 0;
  font-size: 60px;
}

#calculator {
  font-family: "Lucida Sans";
  border-radius: 15px;
  overflow: hidden;
  max-width: 500px;
  background-color: rgb(31, 31, 31);
  margin-top: 10px; 
}

#keys {
  display: grid;
  grid-template-columns: repeat(4, 1fr);
  padding: 25px;
  gap: 10px;
}

#display {
  width: 100%;
  padding: 25px;
  color: white;
  background-color: rgb(23, 15, 0);
  font-size: 4rem;
  text-align: left;
  border: none;
}

button {
  width: 100px;
  height: 100px;
  border-radius: 50%;
  border: none;
  background-color: rgb(48, 45, 45);
  color: white;
  font-size: 3rem;
  font-weight: bold;
  transition: 0.6 ease-in-out;
}

button:hover {
  background-color: rgb(69, 69, 69);
  cursor: pointer;
  transition: 0.8 ease-in;
}

button:active {
  background-color: rgb(130, 130, 130);
  transition: 0.3 ease;
}

.op {
  background-color: rgb(247, 160, 0);
}

.op:hover {
  background-color: rgb(255, 182, 45);
}

.op:active {
  background-color: rgb(255, 207, 118);
}

footer {
  background-color: rgba(0, 0, 0, 0.9); 
  color: white;
  padding: 10px;
  text-align: center;
  width: 100%; 
  position: fixed;
  bottom: 0; 
}