:root {
  --bg-app: #0b0f19;
  --bg-card: rgba(22, 30, 46, 0.88);
  --border-card: rgba(255, 255, 255, 0.08);
  --bg-display: #0f172a;
  --text-main: #f8fafc;
  --text-secondary: #94a3b8;
  --text-dim: #64748b;
  
  /* Button Themes */
  --btn-num-bg: #1e293b;
  --btn-num-hover: #2d3b53;
  --btn-num-active: #334155;
  --btn-num-text: #f1f5f9;

  --btn-fn-bg: #334155;
  --btn-fn-hover: #475569;
  --btn-fn-active: #506179;
  --btn-fn-text: #e2e8f0;

  --btn-sci-bg: #162032;
  --btn-sci-hover: #22324e;
  --btn-sci-active: #2b3e5f;
  --btn-sci-text: #93c5fd;

  --btn-op-bg: #2563eb;
  --btn-op-hover: #1d4ed8;
  --btn-op-active: #1e40af;
  --btn-op-text: #ffffff;
  --btn-op-highlight: #60a5fa;

  --btn-eq-bg: #059669;
  --btn-eq-hover: #047857;
  --btn-eq-active: #065f46;
  --btn-eq-text: #ffffff;

  --shadow-card: 0 25px 50px -12px rgba(0, 0, 0, 0.55), 0 0 0 1px rgba(255, 255, 255, 0.06);
  --shadow-btn: 0 2px 4px rgba(0, 0, 0, 0.2);
  --radius-card: 24px;
  --radius-btn: 12px;
  --font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, "Helvetica Neue", Arial, sans-serif;
}

* {
  box-sizing: border-box;
  margin: 0;
  padding: 0;
  -webkit-tap-highlight-color: transparent;
  user-select: none;
}

body {
  min-height: 100vh;
  display: flex;
  flex-direction: column;
  justify-content: center;
  align-items: center;
  background: radial-gradient(circle at 50% 15%, #1e293b 0%, var(--bg-app) 100%);
  font-family: var(--font-family);
  color: var(--text-main);
  padding: 1.25rem;
  overflow-x: hidden;
}

.calculator-wrapper {
  width: 100%;
  max-width: 720px;
  margin: auto;
}

.calculator-card {
  background: var(--bg-card);
  backdrop-filter: blur(20px);
  -webkit-backdrop-filter: blur(20px);
  border: 1px solid var(--border-card);
  border-radius: var(--radius-card);
  box-shadow: var(--shadow-card);
  padding: 1.5rem;
  display: flex;
  flex-direction: column;
  gap: 1.25rem;
}

/* Header */
.calculator-header {
  display: flex;
  justify-content: space-between;
  align-items: center;
  padding: 0 0.25rem;
}

.header-left {
  display: flex;
  align-items: center;
  gap: 0.5rem;
}

.app-title {
  font-size: 0.875rem;
  font-weight: 600;
  letter-spacing: 0.05em;
  text-transform: uppercase;
  color: var(--text-secondary);
  display: flex;
  align-items: center;
  gap: 0.5rem;
}

.app-title-badge {
  font-size: 0.65rem;
  padding: 0.15rem 0.5rem;
  background: rgba(59, 130, 246, 0.2);
  color: #60a5fa;
  border-radius: 9999px;
  border: 1px solid rgba(59, 130, 246, 0.35);
  font-weight: 700;
}

/* DEG / RAD Mode Toggle Group */
.mode-toggle-group {
  display: inline-flex;
  background: #0f172a;
  padding: 3px;
  border-radius: 9999px;
  border: 1px solid rgba(255, 255, 255, 0.08);
}

.mode-toggle-btn {
  background: transparent;
  border: none;
  outline: none;
  color: var(--text-secondary);
  font-family: inherit;
  font-size: 0.72rem;
  font-weight: 700;
  letter-spacing: 0.06em;
  padding: 0.25rem 0.75rem;
  border-radius: 9999px;
  cursor: pointer;
  transition: all 0.15s ease;
}

.mode-toggle-btn:hover {
  color: var(--text-main);
}

.mode-toggle-btn.is-active {
  background: #2563eb;
  color: #ffffff;
  box-shadow: 0 1px 4px rgba(37, 99, 235, 0.4);
}

.mode-toggle-btn:focus-visible {
  outline: 2px solid #60a5fa;
}

/* Display */
.calculator-display {
  background: var(--bg-display);
  border: 1px solid rgba(255, 255, 255, 0.06);
  border-radius: 16px;
  padding: 1rem 1.25rem;
  text-align: right;
  display: flex;
  flex-direction: column;
  justify-content: flex-end;
  min-height: 108px;
  box-shadow: inset 0 2px 8px rgba(0, 0, 0, 0.4);
  overflow: hidden;
}

.display-top-bar {
  display: flex;
  justify-content: space-between;
  align-items: center;
  min-height: 1.5rem;
  margin-bottom: 0.25rem;
}

.angle-mode-indicator {
  font-size: 0.7rem;
  font-weight: 700;
  letter-spacing: 0.08em;
  color: #60a5fa;
  background: rgba(59, 130, 246, 0.15);
  border: 1px solid rgba(59, 130, 246, 0.25);
  padding: 0.1rem 0.45rem;
  border-radius: 4px;
}

.display-expression {
  color: var(--text-secondary);
  font-size: 1rem;
  word-break: break-all;
  white-space: nowrap;
  overflow: hidden;
  text-overflow: ellipsis;
  letter-spacing: 0.03em;
  font-variant-numeric: tabular-nums;
  transition: opacity 0.2s ease;
  margin-left: auto;
}

.display-main {
  color: var(--text-main);
  font-size: 2.5rem;
  font-weight: 500;
  line-height: 1.15;
  letter-spacing: -0.02em;
  font-variant-numeric: tabular-nums;
  word-break: break-all;
  transition: font-size 0.15s ease, color 0.15s ease;
}

.display-main.error {
  font-size: 1.35rem;
  color: #f87171;
  font-weight: 500;
}

.display-main.shrink-sm {
  font-size: 2rem;
}

.display-main.shrink-md {
  font-size: 1.6rem;
}

.display-main.shrink-lg {
  font-size: 1.25rem;
}

/* Keypad Panels Container */
.calculator-panels {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: 1rem;
  align-items: stretch;
}

.keypad-grid {
  display: grid;
  grid-template-columns: repeat(4, 1fr);
  gap: 0.65rem;
}

/* Base Button */
.btn {
  position: relative;
  border: none;
  outline: none;
  cursor: pointer;
  height: 52px;
  border-radius: var(--radius-btn);
  font-family: inherit;
  font-size: 1.15rem;
  font-weight: 500;
  display: flex;
  justify-content: center;
  align-items: center;
  transition: transform 0.08s ease, background-color 0.15s ease, box-shadow 0.15s ease, border-color 0.15s ease;
  box-shadow: var(--shadow-btn);
}

.btn:active {
  transform: scale(0.96);
}

.btn:focus-visible {
  outline: 2px solid #60a5fa;
  outline-offset: 2px;
}

/* Scientific Buttons */
.btn-sci {
  background-color: var(--btn-sci-bg);
  color: var(--btn-sci-text);
  border: 1px solid rgba(255, 255, 255, 0.06);
  font-size: 1.05rem;
  font-weight: 500;
}

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

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

.btn-const {
  color: #38bdf8;
  font-family: "Times New Roman", Times, serif;
  font-style: italic;
  font-size: 1.25rem;
}

/* Number Buttons */
.btn-num {
  background-color: var(--btn-num-bg);
  color: var(--btn-num-text);
  border: 1px solid rgba(255, 255, 255, 0.05);
}

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

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

/* Function Buttons (AC, ⌫, %, +/-) */
.btn-fn {
  background-color: var(--btn-fn-bg);
  color: var(--btn-fn-text);
  border: 1px solid rgba(255, 255, 255, 0.08);
  font-weight: 500;
}

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

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

/* Operator Buttons (+, -, ×, ÷, ^) */
.btn-op {
  background-color: var(--btn-op-bg);
  color: var(--btn-op-text);
  font-size: 1.35rem;
  border: 1px solid rgba(255, 255, 255, 0.1);
}

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

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

.btn-op.is-active {
  box-shadow: 0 0 0 2px #93c5fd, 0 4px 12px rgba(37, 99, 235, 0.6);
  background-color: var(--btn-op-hover);
}

/* Equals Button */
.btn-eq {
  background-color: var(--btn-eq-bg);
  color: var(--btn-eq-text);
  font-size: 1.45rem;
  border: 1px solid rgba(255, 255, 255, 0.12);
  box-shadow: 0 4px 12px rgba(5, 150, 105, 0.35);
}

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

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

/* Keypad Indicator Footer */
.calculator-shortcuts {
  margin-top: 0.5rem;
  padding: 0.75rem 1rem;
  background: rgba(15, 23, 42, 0.6);
  border-radius: 12px;
  border: 1px solid rgba(255, 255, 255, 0.04);
  font-size: 0.75rem;
  color: var(--text-dim);
  text-align: center;
  line-height: 1.5;
}

.shortcuts-keys {
  display: flex;
  flex-wrap: wrap;
  justify-content: center;
  gap: 0.4rem;
  margin-top: 0.35rem;
}

.key-pill {
  background: rgba(255, 255, 255, 0.08);
  padding: 0.1rem 0.4rem;
  border-radius: 4px;
  font-family: monospace;
  font-size: 0.7rem;
  color: var(--text-secondary);
}

/* Responsive adjustments */
@media (max-width: 640px) {
  body {
    padding: 0.75rem;
  }

  .calculator-wrapper {
    max-width: 440px;
  }

  .calculator-card {
    padding: 1.15rem;
    border-radius: 20px;
    gap: 0.85rem;
  }

  .calculator-panels {
    grid-template-columns: 1fr;
    gap: 0.75rem;
  }

  .keypad-grid {
    gap: 0.5rem;
  }

  .btn {
    height: 46px;
    font-size: 1.1rem;
  }

  .btn-sci {
    height: 40px;
    font-size: 0.95rem;
  }

  .display-main {
    font-size: 2.15rem;
  }

  .calculator-display {
    min-height: 95px;
    padding: 0.85rem 1rem;
  }
}