:root {
  --bg-color: #ffffff;
  --text-color: #000;
  --btn-bg: #f2f2f2;
  --btn-color: #000;
  --operator-bg: #e0e0e0;
  --ac-color: #ff4d4d;
}

.dark-mode {
  --bg-color: #1e1e1e;
  --text-color: #fff;
  --btn-bg: #2a2a2a;
  --btn-color: #fff;
  --operator-bg: #333;
  --ac-color: #ff4d4d;
}

* {
  box-sizing: border-box;
}

body {
  background-color: var(--bg-color);
  font-family: Arial, sans-serif;
  display: flex;
  justify-content: center;
  align-items: center;
  height: 100vh;
  margin: 0;
}

.calculator {
  width: 100%;
  max-width: 350px;
  background: var(--bg-color);
  border-radius: 25px;
  padding: 20px;
  box-shadow: 0 5px 20px rgba(0,0,0,0.2);
}

.theme-toggle {
  display: flex;
  justify-content: flex-end;
}

#themeBtn {
  background: transparent;
  border: none;
  font-size: 1.5rem;
  cursor: pointer;
  color: var(--text-color);
}

.display {
  text-align: right;
  margin: 20px 0;
  min-height: 60px;
}

#operation {
  font-size: 1rem;
  color: gray;
  min-height: 20px;
}

#result {
  font-size: 2rem;
  font-weight: bold;
  color: var(--text-color);
}

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

button {
  background: var(--btn-bg);
  color: var(--btn-color);
  border: none;
  border-radius: 50%;
  font-size: 1.2rem;
  height: 60px;
  width: 60px;
  cursor: pointer;
  transition: 0.2s;
}

button:hover {
  filter: brightness(0.9);
}

.operator {
  background: var(--operator-bg);
}

.ac {
  color: var(--ac-color);
}

.equal {
  background: linear-gradient(135deg, #ff5f6d, #ff8e53);
  color: white;
  grid-row: span 2;
  height: 135px;
  border-radius: 30px;
}

@media (max-width: 400px) {
  .buttons {
    gap: 10px;
  }
  button {
    height: 50px;
    width: 50px;
    font-size: 1rem;
  }
  .equal {
    height: 110px;
  }
}