:root {
  --bg: #f4f4f2;
  --panel: #ffffff;
  --text: #111111;
  --muted: #666666;
  --border: #d6d6d6;
  --warning: #a52222;
  --content-max: 720px;
  --equation-size: 46px;
}

* {
  box-sizing: border-box;
}

html,
body {
  height: 100%;
  margin: 0;
  padding: 0;
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Helvetica, Arial, sans-serif;
  background: var(--bg);
  color: var(--text);
}

.app {
  height: 100%;
  width: 100%;
  max-width: 760px;
  margin: 0 auto;
  padding: 20px;
}

.screen {
  height: 100%;
  display: flex;
  flex-direction: column;
}

.hidden {
  display: none;
}

.settings-card,
.end-card {
  width: 100%;
  max-width: var(--content-max);
  margin: auto;
  padding: 24px 28px;
  border: 1px solid var(--border);
  background: var(--panel);
}

.title {
  margin: 0 0 12px 0;
  font-size: 24px;
  font-weight: 700;
}

.description {
  margin: 0 0 10px 0;
  font-size: 14px;
  line-height: 1.4;
  color: var(--muted);
}

.description a {
  color: inherit;
}

.section {
  margin-top: 16px;
}

.section-title {
  font-size: 14px;
  font-weight: 600;
  margin-bottom: 8px;
}

.op-row {
  padding: 8px 0;
  border-top: 1px solid var(--border);
}

.op-row:first-of-type {
  border-top: 1px solid var(--border);
}

.op-check {
  display: flex;
  align-items: center;
  gap: 8px;
  font-size: 15px;
  font-weight: 600;
}

.op-details {
  margin-top: 6px;
  font-size: 14px;
  color: var(--muted);
  display: flex;
  align-items: center;
  flex-wrap: wrap;
  gap: 6px;
}

.op-label {
  font-weight: 600;
}

.op-range {
  display: flex;
  align-items: center;
  flex-wrap: wrap;
  gap: 6px;
}

.op-details input[type="number"] {
  width: 68px;
  padding: 4px 6px;
  font-size: 14px;
  border: 1px solid var(--border);
}

.op-row.is-disabled {
  opacity: 0.5;
}

.op-row.is-disabled .op-details input[type="number"] {
  background: #f0f0f0;
}

.note {
  font-style: normal;
}

.duration-row,
.delay-row {
  display: flex;
  align-items: center;
  gap: 10px;
  font-size: 14px;
  padding-top: 12px;
  border-top: 1px solid var(--border);
}

.duration-row select,
.delay-row input {
  padding: 6px 8px;
  font-size: 14px;
  border: 1px solid var(--border);
  background: #ffffff;
}

.start-row {
  display: flex;
  align-items: center;
  gap: 12px;
  margin-top: 18px;
}

.start-row .primary {
  margin-left: auto;
}

.warning {
  font-size: 12px;
  color: var(--warning);
}

.primary {
  padding: 8px 20px;
  font-size: 14px;
  font-weight: 600;
  border: 1px solid var(--border);
  background: #f0f0f0;
  cursor: pointer;
}

.primary:disabled {
  cursor: not-allowed;
  opacity: 0.6;
}

#game-screen {
  gap: 10px;
}

.hud {
  display: flex;
  justify-content: space-between;
  font-size: 16px;
  font-weight: 600;
}

.history-panel {
  flex: 1;
  min-height: 0;
  border: 1px solid var(--border);
  background: #fafafa;
  padding: 10px;
  overflow-y: auto;
}

.history-list {
  min-height: 100%;
  max-width: var(--content-max);
  margin: 0 auto;
  display: flex;
  flex-direction: column;
  justify-content: flex-end;
  gap: 6px;
}

.history-item {
  font-size: var(--equation-size);
  font-weight: 700;
  text-align: center;
  line-height: 1.1;
}

.history-item.is-recent {
  color: var(--text);
  opacity: 1;
}

.history-item.is-old {
  color: var(--muted);
  opacity: 0.55;
}

.problem-panel {
  max-width: var(--content-max);
  margin: 0 auto;
  width: 100%;
  border: 1px solid var(--border);
  background: var(--panel);
  padding: 16px;
  text-align: center;
}

.equationLine {
  display: inline-flex;
  align-items: baseline;
  gap: 8px;
  font-size: var(--equation-size);
  font-weight: 700;
  line-height: 1.1;
}

.problem-left {
  white-space: nowrap;
}

.answer-slot {
  position: relative;
  display: inline-block;
  font: inherit;
  line-height: inherit;
  width: 2ch;
  min-width: 2ch;
  box-shadow: inset 0 -1px 0 transparent;
}

.answer-slot:focus-within {
  box-shadow: inset 0 -1px 0 var(--border);
}

.answer-mirror {
  display: inline-block;
  font: inherit;
  line-height: inherit;
  white-space: pre;
}

.answer-input {
  position: absolute;
  inset: 0;
  width: 100%;
  height: 100%;
  padding: 0;
  margin: 0;
  border: 0;
  background: transparent;
  font: inherit;
  line-height: inherit;
  text-align: left;
  color: var(--text);
  outline: none;
  opacity: 0;
  min-width: 0;
  min-height: 0;
  appearance: none;
  -webkit-appearance: none;
}

.final-row {
  display: flex;
  justify-content: space-between;
  font-size: 18px;
  margin-bottom: 12px;
}

@media (max-width: 520px) {
  :root {
    --equation-size: 38px;
  }

  .app {
    padding: 16px;
  }

  .op-details input[type="number"] {
    width: 60px;
  }
}