:root {
    --bg: #f6f7fb;
    --card: #ffffff;
    --text: #1f2937;
    --muted: #6b7280;
    --border: #e5e7eb;
    --accent: #4f46e5;
    --correct: #16a34a;
    --wrong: #dc2626;
  }

  body {
    margin: 0;
    font-family: system-ui, -apple-system, sans-serif;
    background: var(--bg);
    color: var(--text);
    display: flex;
    justify-content: center;
  }

  .container {
    width: min(1000px, 92vw);
    margin: 40px 0;
  }

  header {
    background: var(--card);
    padding: 20px 24px;
    border: 1px solid var(--border);
    border-radius: 14px;
    margin-bottom: 20px;
    box-shadow: 0 4px 12px rgba(0,0,0,0.04);
  }

  h2 { margin: 0; font-size: 20px; font-weight: 700; }

  .stem {
    margin-top: 8px;
    padding: 6px 12px;
    font-size: 12px;
    background: #eef2ff;
    color: var(--accent);
    border-radius: 999px;
    font-weight: 600;
    display: inline-block;
  }

  table {
    width: 100%;
    border-collapse: collapse;
    background: var(--card);
    border-radius: 14px;
    overflow: hidden;
    border: 1px solid var(--border);
    box-shadow: 0 4px 12px rgba(0,0,0,0.04);
    table-layout: fixed;
  }

  th {
    background: #f9fafb;
    text-transform: uppercase;
    font-size: 12px;
    letter-spacing: 0.08em;
    color: var(--muted);
    padding: 15px;
  }

  td {
    border-bottom: 1px solid var(--border);
    padding: 12px;
    text-align: center;
  }

  .case-label {
    background: #f9fafb;
    font-weight: 600;
    color: var(--muted);
    width: 150px;
  }

  input {
    width: 100%;
    padding: 10px;
    font-size: 15px;
    text-align: center;
    border: 1px solid var(--border);
    border-radius: 8px;
    outline: none;
    background: #fff;
    box-sizing: border-box;
  }

  input:focus {
    border-color: var(--accent);
    box-shadow: 0 0 0 3px rgba(79,70,229,0.15);
  }

  .feedback { margin-top: 4px; font-size: 12px; min-height: 14px; }
  .correct { color: var(--correct); font-weight: 600; }
  .wrong { color: var(--wrong); font-weight: 600; }

  .controls { display: flex; gap: 10px; margin-top: 16px; }
  button {
    padding: 10px 20px;
    font-size: 14px;
    border: none;
    border-radius: 10px;
    cursor: pointer;
  }
  button:first-child { background: var(--accent); color: white; }
  button:last-child { background: #e5e7eb; }
.ad {
    background: var(--card);
    border: 1px solid var(--border);
    padding: 10px;
    border-radius: 10px;
    font-size: 13px;
    font-weight: 600;
    color: var(--accent);
    flex: 1;
    cursor: pointer;
    transition: all 0.2s;
    width: calc(20%-10px);
    margin: 10px 0px 10px 0px;
}
.ad:hover{
    background: var(--bg);
}