:root {
  --primary-color: rgba(255, 255, 255, 0.1);
  --secondary-color: rgba(255, 255, 255, 0.2);
  --accent-color: #e74c3c;
  --equal-color: #2ecc71;
  --text-color: white;
  --button-color: rgba(255, 255, 255, 0.4);
  --backdrop-filter: blur(15px);
}

body {
  display: flex;
  justify-content: center;
  align-items: center;
  height: 100vh;
  background: linear-gradient(135deg, #5161a8, #8a67ad);
  font-family: Arial, sans-serif;
  margin: 0;
  flex-direction: column;
}

body.dark-mode {
  --primary-color: rgba(0, 0, 0, 0.3);
  --secondary-color: rgba(0, 0, 0, 0.4);
  --accent-color: #ff6b6b;
  --equal-color: #4caf50;
  --text-color: #ffffff;
  --button-color: rgba(0, 0, 0, 0.5);
}

.calculator {
  background: var(--primary-color);
  padding: 20px;
  border-radius: 15px;
  backdrop-filter: var(--backdrop-filter);
  box-shadow: 0px 4px 15px rgba(0, 0, 0, 0.3);
  width: 320px;
  text-align: center;
}

.display {
  width: 90%;
  height: 60px;
  font-size: 1.8em;
  text-align: right;
  padding: 10px;
  border: none;
  border-radius: 10px;
  margin-bottom: 10px;
  background: var(--secondary-color);
  color: var(--text-color);
}

.history {
  font-size: 0.9em;
  color: lightgray;
  text-align: right;
  margin-bottom: 5px;
  min-height: 20px;
}

.buttons {
  display: grid;
  grid-template-columns: repeat(4, 1fr);
  gap: 10px;
}

button {
  padding: 20px;
  font-size: 1.2em;
  border: none;
  border-radius: 10px;
  cursor: pointer;
  transition: 0.3s;
  background: var(--button-color);
  color: var(--text-color);
}

button.operator {
  background: var(--accent-color);
}

button.equal {
  background: var(--equal-color);
}

button:hover {
  opacity: 0.7;
}

.theme-toggle {
  position: absolute;
  top: 20px;
  right: 20px;
  background: none;
  border: none;
  font-size: 1.5em;
  cursor: pointer;
  color: var(--text-color);
}