* {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
}

body {
  font-family: 'Roboto', sans-serif;
  background: linear-gradient(-45deg, #c9d6ff, #e2e2e2);
  background-size: 400% 400%;
  animation: bg 10s ease infinite;
  display: flex;
  justify-content: center;
  align-items: center;
  height: 100vh;
}
@keyframes bg {
  0% { background-position: 0% 50%; }
  50% { background-position: 100% 50%; }
  100% { background-position: 0% 50%; }
}

.container {
  background: #f9f9f9;
  padding: 2rem;
  border-radius: 20px;
  box-shadow: 0 10px 20px rgba(0,0,0,0.15);
  max-width: 350px;
  width: 100%;
}

h1 {
  text-align: center;
  color: #26415E;
  margin-bottom: 1rem;
}

input#display {
  width: 100%;
  padding: 1rem;
  font-size: 1.5rem;
  border: 2px solid #26415E;
  border-radius: 10px;
  margin-bottom: 1.5rem;
  background-color: #eafff5;
  color: #333;
}

.sub-container {
  display: grid;
  grid-template-columns: repeat(4, 1fr);
  gap: 1rem;
}

.btn {
  padding: 1rem;
  font-size: 1.2rem;
  border: none;
  border-radius: 50px;
  background-color: #f0f0f0;
  cursor: pointer;
  transition: all 0.2s ease-in-out;
}

.btn:hover {
  background-color: #dcefff;
  transform: scale(1.05);
}

.operators {
  background-color: #ffecb3;
}

.equals {
  background-color: #b2fab4;
}