/* ============================================================
   Tokens
   ============================================================ */
:root {
  --body-case: #1b1e24;
  --case-edge: #23272f;

  --display-bg: #10131a;
  --display-text: #f2f5f8;
  --display-sub: #5c6675;
  --display-error: #ff5470;

  --key-bg: #262a33;
  --key-bg-hover: #2f3540;
  --key-bg-active: #363d4a;
  --key-text: #eef1f5;

  --fn-bg: #343a45;
  --fn-text: #cdd4de;

  --op-bg: #ff9f43;
  --op-bg-hover: #ffad5c;
  --op-text: #1b1e24;
  --op-active-ring: #ffd199;

  --equals-bg: #2dd4a7;
  --equals-bg-hover: #45dcb3;
  --equals-text: #0c1512;

  --font-display: "Space Grotesk", system-ui, sans-serif;
  --font-mono: "IBM Plex Mono", ui-monospace, "SF Mono", Menlo, monospace;

  --radius-case: 26px;
  --radius-key: 14px;
}

* {
  box-sizing: border-box;
}

@media (prefers-reduced-motion: reduce) {
  * {
    transition-duration: 0.001ms !important;
  }
}

html, body {
  height: 100%;
}

body {
  margin: 0;
  min-height: 100vh;
  display: flex;
  align-items: center;
  justify-content: center;
  padding: 32px 16px;
  font-family: var(--font-display);
  background:
    radial-gradient(900px 500px at 20% -10%, rgba(255, 159, 67, 0.08), transparent 60%),
    radial-gradient(900px 500px at 90% 110%, rgba(45, 212, 167, 0.08), transparent 60%),
    #0b0d11;
  color: var(--key-text);
}

/* ============================================================
   Calculator case
   ============================================================ */
.calculator {
  width: 100%;
  max-width: 340px;
  background: linear-gradient(180deg, var(--case-edge), var(--body-case));
  border: 1px solid rgba(255, 255, 255, 0.06);
  border-radius: var(--radius-case);
  padding: 20px;
  box-shadow: 0 30px 70px -24px rgba(0, 0, 0, 0.65);
}

/* ============================================================
   Screen
   ============================================================ */
.screen {
  background: var(--display-bg);
  border: 1px solid rgba(255, 255, 255, 0.05);
  border-radius: 16px;
  padding: 20px 18px 16px;
  margin-bottom: 18px;
  text-align: right;
  overflow: hidden;
}

.screen__expression {
  font-family: var(--font-mono);
  font-size: 13px;
  color: var(--display-sub);
  min-height: 16px;
  letter-spacing: 0.01em;
  white-space: nowrap;
  overflow-x: auto;
}

.screen__display {
  font-family: var(--font-mono);
  font-size: 40px;
  font-weight: 500;
  color: var(--display-text);
  margin-top: 6px;
  line-height: 1.1;
  white-space: nowrap;
  overflow-x: auto;
  letter-spacing: -0.01em;
}

.screen__display.is-error {
  color: var(--display-error);
  font-size: 26px;
}

/* ============================================================
   Keys — CSS Grid
   ============================================================ */
.keys {
  display: grid;
  grid-template-columns: repeat(4, 1fr);
  gap: 10px;
}

.key {
  appearance: none;
  border: none;
  border-radius: var(--radius-key);
  padding: 18px 0;
  font-family: var(--font-display);
  font-size: 18px;
  font-weight: 600;
  color: var(--key-text);
  background: var(--key-bg);
  cursor: pointer;
  transition: background 0.12s ease, transform 0.05s ease, box-shadow 0.12s ease;
}

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

.key:active {
  background: var(--key-bg-active);
  transform: translateY(1px);
}

.key:focus-visible {
  outline: 2px solid var(--equals-bg);
  outline-offset: 2px;
}

.key--fn {
  background: var(--fn-bg);
  color: var(--fn-text);
  font-size: 16px;
}

.key--fn:hover {
  background: #3d4451;
}

.key--op {
  background: var(--op-bg);
  color: var(--op-text);
  font-size: 20px;
}

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

.key--op.is-selected {
  box-shadow: 0 0 0 3px var(--op-active-ring) inset;
}

.key--equals {
  background: var(--equals-bg);
  color: var(--equals-text);
  font-size: 20px;
}

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

.key--wide {
  grid-column: span 2;
}

.key--zero {
  grid-column: span 2;
  text-align: center;
}

@media (max-width: 380px) {
  .screen__display {
    font-size: 32px;
  }

  .key {
    padding: 15px 0;
    font-size: 16px;
  }
}