:root {
  --bg-gradient: linear-gradient(135deg, #0f172a, #1f2937);
  --text-color: #e5e7eb;
  --panel-bg: rgba(17, 24, 39, 0.8);
  --panel-border: rgba(255,255,255,0.08);
  --display-bg: linear-gradient(180deg, rgba(31,41,55,0.9), rgba(17,24,39,0.8));
  --muted: #9ca3af;
  --key-bg: #374151;
  --key-op: #2563eb;
  --key-fn: #6b7280;
  --key-eq: #10b981;
  --focus: #93c5fd;
}

@media (prefers-color-scheme: light) {
  :root {
    --bg-gradient: linear-gradient(135deg, #eef2ff, #e2e8f0);
    --text-color: #0f172a;
    --panel-bg: rgba(255, 255, 255, 0.85);
    --panel-border: rgba(0,0,0,0.08);
    --display-bg: linear-gradient(180deg, rgba(241,245,249,0.95), rgba(226,232,240,0.85));
    --muted: #475569;
    --key-bg: #cbd5e1;
    --key-op: #3b82f6;
    --key-fn: #94a3b8;
    --key-eq: #22c55e;
    --focus: #2563eb;
  }
}

[data-theme="light"] {
  --bg-gradient: linear-gradient(135deg, #eef2ff, #e2e8f0);
  --text-color: #0f172a;
  --panel-bg: rgba(255, 255, 255, 0.85);
  --panel-border: rgba(0,0,0,0.08);
  --display-bg: linear-gradient(180deg, rgba(241,245,249,0.95), rgba(226,232,240,0.85));
  --muted: #475569;
  --key-bg: #cbd5e1;
  --key-op: #3b82f6;
  --key-fn: #94a3b8;
  --key-eq: #22c55e;
  --focus: #2563eb;
}

[data-theme="dark"] {
  --bg-gradient: linear-gradient(135deg, #0f172a, #1f2937);
  --text-color: #e5e7eb;
  --panel-bg: rgba(17, 24, 39, 0.8);
  --panel-border: rgba(255,255,255,0.08);
  --display-bg: linear-gradient(180deg, rgba(31,41,55,0.9), rgba(17,24,39,0.8));
  --muted: #9ca3af;
  --key-bg: #374151;
  --key-op: #2563eb;
  --key-fn: #6b7280;
  --key-eq: #10b981;
  --focus: #93c5fd;
}

* { box-sizing: border-box; }
html, body { height: 100%; }
body {
  margin: 0;
  font-family: 'Inter', system-ui, -apple-system, Segoe UI, Roboto, Helvetica, Arial, sans-serif;
  background: var(--bg-gradient);
  color: var(--text-color);
  display: grid;
  place-items: center;
}

.app { width: min(92vw, 420px); }

.calculator {
  background: var(--panel-bg);
  border: 1px solid var(--panel-border);
  border-radius: 16px;
  backdrop-filter: blur(8px);
  box-shadow: 0 10px 30px rgba(0,0,0,0.2);
  overflow: hidden;
}

.calc-header {
  display: flex;
  align-items: center;
  justify-content: flex-end;
  gap: 8px;
  padding: 10px 12px 0 12px;
}

.theme-toggle {
  display: inline-flex;
  align-items: center;
  gap: 8px;
  border: 1px solid var(--panel-border);
  background: transparent;
  color: var(--text-color);
  border-radius: 999px;
  padding: 6px 10px;
  cursor: pointer;
}
.theme-toggle:focus-visible { outline: 2px solid var(--focus); outline-offset: 2px; }

.display {
  padding: 18px 16px;
  text-align: right;
  background: var(--display-bg);
  border-bottom: 1px solid var(--panel-border);
}
.display__history {
  min-height: 24px;
  color: var(--muted);
  font-size: 0.95rem;
  letter-spacing: 0.3px;
  white-space: nowrap;
  overflow: hidden;
  text-overflow: ellipsis;
}
.display__current {
  font-size: clamp(2rem, 6vw, 2.6rem);
  font-weight: 700;
  line-height: 1.2;
}

.keys {
  display: grid;
  grid-template-columns: repeat(4, 1fr);
  gap: 10px;
  padding: 14px;
}

.key {
  appearance: none;
  border: none;
  border-radius: 12px;
  padding: 16px 12px;
  font-size: 1.15rem;
  font-weight: 600;
  color: var(--text-color);
  background: var(--key-bg);
  box-shadow: inset 0 -2px 0 rgba(0,0,0,0.25);
  cursor: pointer;
  transition: transform 0.02s ease, filter 0.2s ease, background 0.2s ease;
}
.key:hover { filter: brightness(1.08); }
.key:active { transform: translateY(1px); }
.key:focus-visible { outline: 2px solid var(--focus); outline-offset: 2px; }

.key--op { background: var(--key-op); }
.key--fn { background: var(--key-fn); }
.key--equals { background: var(--key-eq); }
.key--wide { grid-column: span 2; }


@media (max-width: 360px) {
  .key { padding: 14px 10px; font-size: 1rem; }
}