:root {
  --bg-color: #0f0f0f;
  --text-color: #ffffff;
  --btn-bg: #222;
  --btn-hover: #444;
  --display-bg: #000;
  --accent: #1abc9c;
}

body.light {
  --bg-color: #f0f0f0;
  --text-color: #111;
  --btn-bg: #e0e0e0;
  --btn-hover: #ccc;
  --display-bg: #ffffff;
  --accent: #3498db;
}

* {
  box-sizing: border-box;
  font-family: 'Segoe UI', sans-serif;
  transition: all 0.3s ease;
}

body {
  margin: 0;
  background-color: var(--bg-color);
  color: var(--text-color);
  display: flex;
  justify-content: center;
  align-items: center;
  height: 100vh;
  flex-direction: column;
}

.container {
  width: 100%;
  max-width: 400px;
  padding: 20px;
}

.theme-toggle {
  display: flex;
  justify-content: flex-end;
  align-items: center;
  margin-bottom: 15px;
  font-size: 14px;
  color: var(--text-color);
}

.mode-label {
  margin-left: 8px;
}

.switch {
  position: relative;
  display: inline-block;
  width: 44px;
  height: 24px;
}

.switch input {
  opacity: 0;
  width: 0;
  height: 0;
}

.slider {
  position: absolute;
  cursor: pointer;
  background-color: #ccc;
  border-radius: 24px;
  top: 0;
  left: 0;
  right: 0;
  bottom: 0;
  transition: 0.4s;
}

.slider::before {
  content: "";
  position: absolute;
  height: 18px;
  width: 18px;
  border-radius: 50%;
  background-color: white;
  left: 3px;
  bottom: 3px;
  transition: 0.4s;
}

input:checked + .slider {
  background-color: var(--accent);
}

input:checked + .slider::before {
  transform: translateX(20px);
}

.calculator {
  border-radius: 20px;
  padding: 20px;
  background: var(--btn-bg);
  box-shadow: 0 10px 20px rgba(0,0,0,0.25);
  backdrop-filter: blur(15px);
}

.glass {
  background: rgba(255, 255, 255, 0.05);
  backdrop-filter: blur(20px);
  border: 1px solid rgba(255, 255, 255, 0.1);
}

.display {
  width: 100%;
  height: 60px;
  background: var(--display-bg);
  color: var(--accent);
  font-size: 2rem;
  text-align: right;
  padding: 10px;
  border: none;
  border-radius: 12px;
  margin-bottom: 20px;
  transition: all 0.3s ease;
}

.buttons {
  display: grid;
  grid-template-columns: repeat(4, 1fr);
  gap: 12px;
}

.btn {
  padding: 18px;
  font-size: 1.2rem;
  border: none;
  border-radius: 12px;
  background-color: var(--btn-bg);
  color: var(--text-color);
  cursor: pointer;
  box-shadow: inset 0 0 0 transparent;
  transition: transform 0.2s, box-shadow 0.2s;
}

.btn:hover {
  background-color: var(--btn-hover);
}

.btn:active {
  transform: scale(0.95);
  box-shadow: inset 0 0 10px var(--accent);
}

.equals {
  background-color: var(--accent);
  color: white;
  grid-row: span 2;
}

.zero {
  grid-column: span 2;
}