:root {
  /* Theme variables (default = dark theme) */
  --bg-1: #0f1724;
  --bg-2: #05263a;
  --card: rgba(255, 255, 255, 0.06);
  --accent: linear-gradient(135deg, #6ee7b7 0%, #3b82f6 100%);
  --glass: rgba(255, 255, 255, 0.04);
  --muted: rgba(255, 255, 255, 0.6);
  --danger: #ff6b6b;
  --shadow: 0 8px 30px rgba(2, 6, 23, 0.6);

  --text: #f8fafc;
  --btn-text: #e6eef8;
  --operator-text: #eaf2ff;
  --display-bg: linear-gradient(
    180deg,
    rgba(255, 255, 255, 0.02),
    rgba(255, 255, 255, 0.01)
  );
  --btn-bg: var(--glass);
  --equals-bg: var(--accent);

  font-family:
    Inter,
    ui-sans-serif,
    system-ui,
    -apple-system,
    "Segoe UI",
    Roboto,
    "Helvetica Neue",
    Arial;
}

:root[data-theme="light"] {
  --bg-1: #f5f7fb;
  --bg-2: #e9eef6;
  --card: rgba(2, 6, 23, 0.03);
  --accent: linear-gradient(135deg, #2563eb 0%, #06b6d4 100%);
  --glass: rgba(2, 6, 23, 0.04);
  --muted: rgba(2, 6, 23, 0.5);
  --danger: #e11d48;
  --shadow: 0 8px 28px rgba(2, 6, 23, 0.06);

  --text: #071123;
  --btn-text: #071123;
  --operator-text: #071123;
  --display-bg: linear-gradient(
    180deg,
    rgba(2, 6, 23, 0.02),
    rgba(2, 6, 23, 0.01)
  );
  --btn-bg: rgba(255, 255, 255, 0.92);
  --equals-bg: linear-gradient(135deg, #60a5fa 0%, #06b6d4 100%);
}

:root[data-theme="ocean"] {
  --bg-1: #071a2b;
  --bg-2: #04233a;
  --card: rgba(255, 255, 255, 0.03);
  --accent: linear-gradient(135deg, #00d4ff 0%, #6ee7b7 100%);
  --glass: rgba(255, 255, 255, 0.03);
  --muted: rgba(255, 255, 255, 0.65);
  --danger: #ff7b7b;
  --shadow: 0 10px 34px rgba(4, 12, 20, 0.6);

  --text: #e8fbff;
  --btn-text: #e8fbff;
  --operator-text: #e8fbff;
  --display-bg: linear-gradient(
    180deg,
    rgba(0, 0, 0, 0.04),
    rgba(255, 255, 255, 0.01)
  );
  --btn-bg: rgba(255, 255, 255, 0.02);
  --equals-bg: linear-gradient(135deg, #00d4ff 0%, #6ee7b7 100%);
}

/* Accessibility: visually-hidden label for the theme select */
.visually-hidden {
  position: absolute !important;
  height: 1px;
  width: 1px;
  overflow: hidden;
  clip: rect(1px, 1px, 1px, 1px);
  white-space: nowrap;
}

/* Base reset and layout */
* {
  box-sizing: border-box;
}
html,
body {
  height: 100%;
}
body {
  margin: 0;
  background:
    radial-gradient(
      1200px 600px at 10% 10%,
      rgba(59, 130, 246, 0.12),
      transparent
    ),
    radial-gradient(
      1000px 500px at 90% 90%,
      rgba(110, 231, 183, 0.06),
      transparent
    ),
    linear-gradient(180deg, var(--bg-1), var(--bg-2));
  color: var(--text);
  -webkit-font-smoothing: antialiased;
  -moz-osx-font-smoothing: grayscale;
  display: flex;
  align-items: center;
  justify-content: center;
  padding: 24px;
}

/* App layout */
.app {
  width: 100%;
  max-width: 520px;
}

.calculator {
  background: linear-gradient(
    180deg,
    rgba(255, 255, 255, 0.03),
    rgba(255, 255, 255, 0.02)
  );
  border-radius: 14px;
  padding: 16px;
  box-shadow: var(--shadow);
  backdrop-filter: blur(8px) saturate(120%);
  border: 1px solid rgba(255, 255, 255, 0.04);
  display: grid;
  gap: 12px;
}

/* Toolbar with theme selector */
.toolbar {
  display: flex;
  justify-content: space-between;
  align-items: center;
  gap: 12px;
}
.left-tools {
  display: flex;
  gap: 8px;
  align-items: center;
}
#theme-select {
  background: var(--btn-bg);
  color: var(--btn-text);
  border: none;
  padding: 8px 10px;
  border-radius: 8px;
  font-size: 0.95rem;
  box-shadow: 0 6px 18px rgba(2, 6, 23, 0.25);
  cursor: pointer;
}

/* mini buttons for toolbar */
.mini-btn {
  background: var(--btn-bg);
  color: var(--btn-text);
  border: none;
  padding: 8px 10px;
  border-radius: 8px;
  cursor: pointer;
  box-shadow: 0 6px 14px rgba(2, 6, 23, 0.12);
}
.mini-btn[aria-pressed="true"] {
  box-shadow: 0 8px 18px rgba(59, 130, 246, 0.18);
  transform: translateY(0.5px);
}

/* Scientific panel */
.scientific {
  display: grid;
  grid-template-columns: repeat(4, 1fr);
  gap: 8px;
  transition:
    max-height 0.18s ease,
    opacity 0.18s ease;
  overflow: hidden;
  max-height: 0;
  opacity: 0;
}
.scientific.show {
  max-height: 420px; /* large enough for content */
  opacity: 1;
}

/* Display */
.display {
  background: var(--display-bg);
  padding: 18px 16px;
  border-radius: 10px;
  font-size: 2.1rem;
  text-align: right;
  min-height: 72px;
  display: flex;
  align-items: center;
  justify-content: flex-end;
  word-break: break-all;
  color: var(--text);
  box-shadow: inset 0 -6px 18px rgba(2, 6, 23, 0.4);
}

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

/* Buttons */
.btn {
  background: var(--btn-bg);
  border: none;
  color: var(--btn-text);
  padding: 14px;
  border-radius: 10px;
  font-size: 1rem;
  cursor: pointer;
  transition:
    transform 0.06s ease,
    box-shadow 0.06s ease,
    background 0.12s;
  box-shadow: 0 4px 12px rgba(2, 6, 23, 0.35);
  user-select: none;
}

.btn:active {
  transform: translateY(1px) scale(0.998);
}

.btn.operator {
  background: linear-gradient(
    180deg,
    rgba(59, 130, 246, 0.12),
    rgba(59, 130, 246, 0.08)
  );
  color: var(--operator-text);
}

.btn.function {
  background: linear-gradient(
    180deg,
    rgba(255, 255, 255, 0.02),
    rgba(255, 255, 255, 0.01)
  );
  color: var(--muted);
}

.btn.equals {
  background: var(--equals-bg);
  color: var(--operator-text);
  grid-column: 4 / 5;
  grid-row: span 1;
  box-shadow: 0 8px 24px rgba(59, 130, 246, 0.24);
  font-weight: 600;
}

.btn.sci {
  padding: 10px;
  font-size: 0.92rem;
}

/* Make the zero a bit larger (occupy one cell but visually heavier) */
.btn[data-value="0"] {
  grid-column: 2 / 3;
}

/* Focus styles */
.btn:focus {
  outline: 2px solid rgba(59, 130, 246, 0.24);
  outline-offset: 2px;
}

/* Error shake */
@keyframes shake {
  10%,
  90% {
    transform: translateX(-1px);
  }
  20%,
  80% {
    transform: translateX(2px);
  }
  30%,
  50%,
  70% {
    transform: translateX(-4px);
  }
  40%,
  60% {
    transform: translateX(4px);
  }
}
.display.error {
  animation: shake 0.36s ease;
  background: linear-gradient(
    180deg,
    rgba(255, 80, 80, 0.08),
    rgba(255, 255, 255, 0.01)
  );
  color: #ffd3d3;
}

/* Footer */
.credits {
  text-align: center;
  margin-top: 12px;
  color: var(--muted);
  font-size: 0.8rem;
}

/* Responsive */
@media (max-width: 520px) {
  .app {
    max-width: 420px;
  }
  .display {
    font-size: 1.8rem;
    min-height: 64px;
    padding: 14px;
  }
  .btn {
    padding: 12px;
    font-size: 0.95rem;
  }
  .scientific {
    gap: 6px;
  }
}