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

body {
  font-family: 'Inter', sans-serif;
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
  height: 100vh;
  transition: background-color 0.3s;
}

/* Toggle */
.toggle-btn {
  margin-bottom: 15px;
  padding: 10px 20px;
  font-size: 1rem;
  background: #007bff;
  color: white;
  border: none;
  border-radius: 12px;
  cursor: pointer;
}

/* DARK THEME */
body.dark {
  background-color: #0f0f1c;
}
body.dark .calculator {
  background: linear-gradient(145deg, #121220, #0d0d1a);
  box-shadow: 0 0 30px rgba(0, 128, 255, 0.15);
}
body.dark .display {
  color: white;
}
body.dark .expression {
  color: #aaa;
}
body.dark .btn.gray {
  background-color: #1f1f2e;
  color: white;
}
body.dark .btn.blue {
  background-color: #007bff;
  color: white;
}

/* LIGHT THEME */
body.light {
  background-color: #f2f5fa;
}
body.light .calculator {
  background: #ffffff;
  box-shadow: 0 0 20px rgba(0, 0, 0, 0.08);
}
body.light .display {
  color: #000;
}
body.light .expression {
  color: #555;
}
body.light .btn.gray {
  background-color: #f0f0f0;
  color: #000;
}
body.light .btn.blue {
  background-color: #007bff;
  color: white;
}

/* Calculator container */
.calculator {
  width: 90%;
  max-width: 375px;
  border-radius: 20px;
  padding: 20px;
  transition: all 0.3s ease;
}

/* Display */
.display {
  padding: 20px 10px;
  margin-bottom: 20px;
  text-align: right;
  word-wrap: break-word;
}
.expression {
  font-size: 1rem;
}
.result {
  font-size: 2.2rem;
  font-weight: bold;
  margin-top: 5px;
}

/* Buttons */
.buttons {
  display: grid;
  grid-template-columns: repeat(4, 1fr);
  gap: 15px;
}
.btn {
  padding: 20px;
  font-size: 1.2rem;
  border: none;
  border-radius: 15px;
  cursor: pointer;
  transition: 0.2s ease-in-out;
}
.btn:active {
  transform: scale(0.97);
}
.wide2 {
  grid-column: span 2;
}

/* Extra height for + and = */
.tall {
  grid-row: span 2;
  align-self: stretch;
  padding-top: 25px;
  padding-bottom: 25px;
}


.buttons {
  gap: 12px; /* reduce this to make all buttons tighter */
}

.btn {
  height: 100%;
}

.result.empty {
  color: #888; /* light gray or muted */
}