:root {
  --font-main: 'Pretendard', -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, sans-serif;
  --font-mono: 'JetBrains Mono', monospace;

  /* 다크 테마 기본 변수 */
  --bg-gradient: radial-gradient(circle at 50% 10%, #1e2640 0%, #0d111a 100%);
  --app-bg: rgba(22, 27, 34, 0.75);
  --app-border: rgba(255, 255, 255, 0.12);
  --app-shadow: 0 25px 60px rgba(0, 0, 0, 0.5), 0 0 1px rgba(255, 255, 255, 0.2);

  --text-main: #f0f6fc;
  --text-muted: #8b949e;
  --text-dim: #484f58;

  --key-num-bg: rgba(255, 255, 255, 0.08);
  --key-num-hover: rgba(255, 255, 255, 0.16);
  --key-num-active: rgba(255, 255, 255, 0.24);
  --key-num-text: #f0f6fc;

  --key-action-bg: rgba(255, 255, 255, 0.12);
  --key-action-hover: rgba(255, 255, 255, 0.2);
  --key-action-active: rgba(255, 255, 255, 0.28);
  --key-action-text: #58a6ff;

  --key-op-bg: #1f6feb;
  --key-op-hover: #388bfd;
  --key-op-active: #58a6ff;
  --key-op-text: #ffffff;

  --key-equal-bg: linear-gradient(135deg, #238636 0%, #2ea043 100%);
  --key-equal-hover: linear-gradient(135deg, #2ea043 0%, #3fb950 100%);
  --key-equal-active: #3fb950;
  --key-equal-text: #ffffff;

  --key-func-bg: rgba(88, 166, 255, 0.1);
  --key-func-hover: rgba(88, 166, 255, 0.2);
  --key-func-text: #79c0ff;

  --panel-bg: rgba(13, 17, 23, 0.95);
  --panel-border: rgba(255, 255, 255, 0.1);
  --badge-bg: rgba(56, 139, 253, 0.15);
  --badge-text: #58a6ff;
}

[data-theme="light"] {
  --bg-gradient: radial-gradient(circle at 50% 10%, #e2e8f0 0%, #cbd5e1 100%);
  --app-bg: rgba(255, 255, 255, 0.85);
  --app-border: rgba(0, 0, 0, 0.08);
  --app-shadow: 0 25px 60px rgba(0, 0, 0, 0.12), 0 0 1px rgba(0, 0, 0, 0.1);

  --text-main: #0f172a;
  --text-muted: #64748b;
  --text-dim: #94a3b8;

  --key-num-bg: #ffffff;
  --key-num-hover: #f1f5f9;
  --key-num-active: #e2e8f0;
  --key-num-text: #0f172a;

  --key-action-bg: #e2e8f0;
  --key-action-hover: #cbd5e1;
  --key-action-active: #94a3b8;
  --key-action-text: #2563eb;

  --key-op-bg: #3b82f6;
  --key-op-hover: #2563eb;
  --key-op-active: #1d4ed8;
  --key-op-text: #ffffff;

  --key-equal-bg: linear-gradient(135deg, #10b981 0%, #059669 100%);
  --key-equal-hover: linear-gradient(135deg, #059669 0%, #047857 100%);
  --key-equal-active: #047857;
  --key-equal-text: #ffffff;

  --key-func-bg: #f8fafc;
  --key-func-hover: #e2e8f0;
  --key-func-text: #2563eb;

  --panel-bg: rgba(255, 255, 255, 0.96);
  --panel-border: rgba(0, 0, 0, 0.08);
  --badge-bg: rgba(37, 99, 235, 0.12);
  --badge-text: #2563eb;
}

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

body {
  font-family: var(--font-main);
  background: var(--bg-gradient);
  min-height: 100vh;
  display: flex;
  align-items: center;
  justify-content: center;
  color: var(--text-main);
  padding: 16px;
  overflow-x: hidden;
  transition: background 0.3s ease;
}

/* 계산기 메인 컨테이너 */
.calculator-app {
  position: relative;
  width: 100%;
  max-width: 380px;
  background: var(--app-bg);
  backdrop-filter: blur(20px);
  -webkit-backdrop-filter: blur(20px);
  border: 1px solid var(--app-border);
  border-radius: 28px;
  box-shadow: var(--app-shadow);
  padding: 24px;
  display: flex;
  flex-direction: column;
  gap: 16px;
  transition: max-width 0.3s cubic-bezier(0.4, 0, 0.2, 1), transform 0.2s ease;
  overflow: hidden;
}

.calculator-app.scientific-mode {
  max-width: 520px;
}

/* 헤더 */
.app-header {
  display: flex;
  justify-content: space-between;
  align-items: center;
  padding-bottom: 8px;
  border-bottom: 1px solid var(--app-border);
}

.header-left {
  display: flex;
  align-items: center;
  gap: 8px;
}

.app-title {
  font-size: 0.95rem;
  font-weight: 600;
  letter-spacing: -0.3px;
  color: var(--text-main);
}

.mode-badge {
  font-size: 0.72rem;
  padding: 2px 8px;
  border-radius: 12px;
  background: var(--badge-bg);
  color: var(--badge-text);
  font-weight: 500;
}

.header-actions {
  display: flex;
  align-items: center;
  gap: 4px;
}

.icon-btn {
  background: transparent;
  border: none;
  color: var(--text-muted);
  width: 32px;
  height: 32px;
  border-radius: 8px;
  display: flex;
  align-items: center;
  justify-content: center;
  cursor: pointer;
  transition: all 0.2s ease;
}

.icon-btn:hover {
  background: var(--key-num-hover);
  color: var(--text-main);
}

.icon-btn:active {
  transform: scale(0.92);
}

.icon-btn.active {
  color: var(--key-op-bg);
}

.hidden {
  display: none !important;
}

/* 디스플레이 화면 */
.display-container {
  background: rgba(0, 0, 0, 0.15);
  border-radius: 18px;
  padding: 16px 20px;
  display: flex;
  flex-direction: column;
  justify-content: flex-end;
  align-items: flex-end;
  min-height: 110px;
  cursor: pointer;
  position: relative;
  overflow: hidden;
  border: 1px solid var(--app-border);
  transition: border-color 0.2s;
}

.display-container:hover {
  border-color: rgba(88, 166, 255, 0.4);
}

.expression-line {
  font-family: var(--font-mono);
  font-size: 0.95rem;
  color: var(--text-muted);
  min-height: 1.4em;
  word-break: break-all;
  text-align: right;
  line-height: 1.4;
}

.current-line {
  font-family: var(--font-mono);
  font-size: 2.8rem;
  font-weight: 600;
  color: var(--text-main);
  word-break: break-all;
  text-align: right;
  line-height: 1.15;
  margin-top: 4px;
  transition: font-size 0.15s ease;
}

.copy-tooltip {
  position: absolute;
  top: 10px;
  left: 12px;
  font-size: 0.75rem;
  background: var(--key-op-bg);
  color: #fff;
  padding: 3px 8px;
  border-radius: 6px;
  opacity: 0;
  pointer-events: none;
  transform: translateY(-5px);
  transition: all 0.2s ease;
}

.copy-tooltip.show {
  opacity: 1;
  transform: translateY(0);
}

/* 공학용 상단 툴바 */
.scientific-bar {
  display: flex;
  justify-content: space-between;
  align-items: center;
  padding: 2px 4px;
}

.unit-toggle-btn {
  font-family: var(--font-mono);
  font-size: 0.75rem;
  font-weight: 600;
  padding: 4px 10px;
  border-radius: 8px;
  border: 1px solid var(--app-border);
  background: var(--key-func-bg);
  color: var(--key-func-text);
  cursor: pointer;
  transition: all 0.2s;
}

.unit-toggle-btn.active {
  background: var(--badge-bg);
  border-color: var(--badge-text);
}

.memory-indicator {
  font-size: 0.72rem;
  color: var(--text-muted);
  font-family: var(--font-mono);
}

/* 키패드 그리드 */
.keypad {
  display: flex;
  flex-direction: column;
  gap: 10px;
}

.scientific-keys {
  display: grid;
  grid-template-columns: repeat(5, 1fr);
  gap: 8px;
  padding-bottom: 6px;
  border-bottom: 1px dashed var(--app-border);
  animation: fadeIn 0.25s ease-out;
}

.standard-grid {
  display: grid;
  grid-template-columns: repeat(4, 1fr);
  gap: 10px;
}

/* 버튼 스타일링 */
.key {
  position: relative;
  font-family: var(--font-main);
  height: 60px;
  border-radius: 16px;
  border: 1px solid transparent;
  font-size: 1.35rem;
  font-weight: 500;
  display: flex;
  align-items: center;
  justify-content: center;
  cursor: pointer;
  transition: all 0.15s cubic-bezier(0.2, 0, 0, 1);
  box-shadow: 0 2px 6px rgba(0, 0, 0, 0.08);
}

.key:active, .key.pressed {
  transform: scale(0.93);
}

.num-key {
  background: var(--key-num-bg);
  color: var(--key-num-text);
  border-color: var(--app-border);
}

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

.action-key {
  background: var(--key-action-bg);
  color: var(--key-action-text);
  font-weight: 600;
}

.action-key:hover {
  background: var(--key-action-hover);
}

.op-key {
  background: var(--key-op-bg);
  color: var(--key-op-text);
  font-size: 1.5rem;
}

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

.op-key.active-op {
  background: #ffffff;
  color: var(--key-op-bg);
  border-color: var(--key-op-bg);
}

.equal-key {
  background: var(--key-equal-bg);
  color: var(--key-equal-text);
  font-size: 1.6rem;
}

.equal-key:hover {
  background: var(--key-equal-hover);
}

.func-key {
  height: 44px;
  font-size: 0.95rem;
  background: var(--key-func-bg);
  color: var(--key-func-text);
  border: 1px solid var(--app-border);
  font-family: var(--font-mono);
}

.func-key:hover {
  background: var(--key-func-hover);
}

/* 계산 기록 사이드 패널 */
.history-panel {
  position: absolute;
  top: 0;
  right: 0;
  width: 100%;
  height: 100%;
  background: var(--panel-bg);
  backdrop-filter: blur(24px);
  -webkit-backdrop-filter: blur(24px);
  z-index: 20;
  padding: 20px;
  display: flex;
  flex-direction: column;
  animation: slideInRight 0.25s cubic-bezier(0.16, 1, 0.3, 1);
}

.history-header {
  display: flex;
  justify-content: space-between;
  align-items: center;
  padding-bottom: 12px;
  border-bottom: 1px solid var(--panel-border);
}

.history-header h3 {
  font-size: 1.05rem;
  font-weight: 600;
}

.history-actions {
  display: flex;
  align-items: center;
  gap: 8px;
}

.text-btn {
  background: none;
  border: none;
  color: #f85149;
  font-size: 0.85rem;
  cursor: pointer;
  padding: 4px 8px;
  border-radius: 6px;
}

.text-btn:hover {
  background: rgba(248, 81, 73, 0.15);
}

.history-list {
  flex: 1;
  overflow-y: auto;
  padding: 12px 0;
  display: flex;
  flex-direction: column;
  gap: 12px;
}

.history-item {
  padding: 10px 14px;
  background: var(--key-num-bg);
  border: 1px solid var(--panel-border);
  border-radius: 12px;
  cursor: pointer;
  text-align: right;
  transition: all 0.2s;
}

.history-item:hover {
  background: var(--key-num-hover);
  border-color: rgba(88, 166, 255, 0.3);
  transform: translateX(-3px);
}

.history-expr {
  font-family: var(--font-mono);
  font-size: 0.82rem;
  color: var(--text-muted);
  margin-bottom: 4px;
}

.history-res {
  font-family: var(--font-mono);
  font-size: 1.25rem;
  font-weight: 600;
  color: var(--text-main);
}

.empty-history {
  display: flex;
  align-items: center;
  justify-content: center;
  height: 100%;
  color: var(--text-dim);
  font-size: 0.9rem;
}

/* 푸터 */
.app-footer {
  text-align: center;
  font-size: 0.7rem;
  color: var(--text-dim);
  padding-top: 4px;
}

/* 애니메이션 */
@keyframes fadeIn {
  from { opacity: 0; transform: translateY(-8px); }
  to { opacity: 1; transform: translateY(0); }
}

@keyframes slideInRight {
  from { transform: translateX(100%); }
  to { transform: translateX(0); }
}

/* 반응형 모바일 최적화 */
@media (max-width: 480px) {
  body {
    padding: 0;
    align-items: flex-end;
  }
  .calculator-app {
    max-width: 100% !important;
    border-radius: 28px 28px 0 0;
    min-height: 90vh;
    padding: 20px 16px;
  }
  .key {
    height: 56px;
  }
}