:root {
    --primary: #4f8a8b;
    --primary-dark: #3a6b6c;
    --bg: #0f1419;
    --bg2: #1a2332;
    --bg3: #243044;
    --text: #e8eaed;
    --text-dim: #9aa0a6;
    --border: #2d3a4a;
}
* { margin: 0; padding: 0; box-sizing: border-box; }
body {
    font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, sans-serif;
    background: var(--bg);
    color: var(--text);
    min-height: 100vh;
    padding: 20px;
}
.container { max-width: 720px; margin: 0 auto; }
.header { text-align: center; margin-bottom: 32px; }
.header.no-back-margin { margin-bottom: 0; }
.header h1 { font-size: 1.3rem; color: var(--primary); margin-bottom: 8px; }
.header p { color: var(--text-dim); font-size: 0.9rem; }
.back-link { display: inline-block; margin-bottom: 16px; color: var(--primary); text-decoration: none; font-size: 0.85rem; }
.back-link:hover { text-decoration: underline; }

/* ===== 广告位 ===== */
.ad-space {
    background: var(--bg2);
    border: 1px dashed var(--border);
    border-radius: 8px;
    padding: 16px;
    margin-bottom: 20px;
    min-height: 90px;
    display: flex;
    align-items: center;
    justify-content: center;
    position: relative; /* 让角标相对广告位定位 */
}
.ad-label {
    position: absolute;
    top: 4px;
    left: 4px;
    font-size: 10px;
    color: var(--text-dim);
    opacity: 0.5;
    padding: 2px 6px;
    background: rgba(0,0,0,0.3);
    border-radius: 4px;
    display: inline-block;
    line-height: 1.2;
}
.ad-content {
    width: 100%;
    text-align: center;
}
.ad-content img {
    max-width: 100%;
    height: auto;
    border-radius: 6px;
}

.card {
    background: var(--bg2);
    border: 1px solid var(--border);
    border-radius: 12px;
    padding: 24px;
    margin-bottom: 20px;
}
.card h2 { font-size: 1.1rem; color: var(--primary); margin-bottom: 16px; }

.form-row {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(180px, 1fr));
    gap: 16px;
    margin-bottom: 16px;
}
.form-group label {
    display: block;
    font-size: 0.85rem;
    color: var(--text-dim);
    margin-bottom: 6px;
}
.form-group input, .form-group select {
    width: 100%;
    padding: 10px 12px;
    background: var(--bg3);
    border: 1px solid var(--border);
    border-radius: 8px;
    color: var(--text);
    font-size: 0.95rem;
    outline: none;
}
.form-group input:focus, .form-group select:focus {
    border-color: var(--primary);
}

.btn-row { display: flex; gap: 12px; flex-wrap: wrap; }
.btn {
    padding: 12px 24px;
    border: none;
    border-radius: 8px;
    font-size: 0.95rem;
    font-weight: 600;
    cursor: pointer;
    transition: all 0.2s;
}
.btn-primary { background: var(--primary); color: #fff; }
.btn-primary:hover { background: var(--primary-dark); transform: translateY(-1px); }
.btn-secondary { background: var(--bg3); color: var(--text); border: 1px solid var(--border); }
.btn-secondary:hover { border-color: var(--primary); }
.btn-share { background: var(--bg3); color: var(--text); border: 1px solid var(--border); }
.btn-share:hover { border-color: var(--primary); }
.btn:disabled { opacity: 0.5; cursor: not-allowed; transform: none; }

/* ===== 分享弹窗 ===== */
.share-modal-overlay {
    display: none; position: fixed; inset: 0; background: rgba(0,0,0,0.6); z-index: 1000;
    justify-content: center; align-items: center;
}
.share-modal-overlay.active { display: flex; }
.share-modal {
    background: var(--bg2); border: 1px solid var(--border); border-radius: 12px;
    padding: 24px; max-width: 380px; width: 90%; text-align: center;
}
.share-modal h3 { font-size: 1rem; color: var(--primary); margin-bottom: 16px; }
.share-preview-img {
    width: 100%;
    border-radius: 8px;
    margin-bottom: 16px;
    box-shadow: 0 2px 8px rgba(0,0,0,0.2);
    -webkit-touch-callout: default;
    -webkit-user-select: none;
    user-select: none;
}
.share-close { padding: 8px 20px; border: none; border-radius: 8px; background: var(--primary); color: #fff; font-size: 0.9rem; cursor: pointer; }
.share-close:hover { background: var(--primary-dark); }

.preview {
    background: #fff;
    color: #1a1a1a;
    border-radius: 8px;
    padding: 20px;
    font-family: 'Times New Roman', serif;
    display: none;
    width: 332px;
    margin: 0 auto;
    transform-origin: top center;
    --font-size: 12px;
    --gap-addsub: 12px;
    --gap-mixed: 36px;
    --letter-spacing: 1px;
    --line-height-mixed: 8px;
}
.preview.visible { display: block; }
.preview-page {
    width: 100%;
    aspect-ratio: 210/297;
    background: #fff;
    box-shadow: 0 2px 8px rgba(0,0,0,0.15);
    margin-bottom: 32px;
    padding: 10px 5px;
    position: relative;
}
.preview-page:last-child { margin-bottom: 0; }
.preview-page-header {
    text-align: center;
    font-size: 14px;
    font-weight: bold;
    margin-bottom: 8px;
    padding-bottom: 6px;
    border-bottom: 1px solid #ccc;
    color: #666;
}

/* 答案页区分 header 颜色，答案数字用深蓝突出 */
.preview-page.answer-page .preview-page-header {
    color: #2563eb;
    border-bottom-color: #93c5fd;
}
.preview-page.answer-page .preview-problem {
    color: #1e40af;
}

/* ===== 加法和减法 ===== */
.addsub-grid {
    display: grid;
    gap: var(--gap-addsub);
}
.addsub-grid .preview-problem {
    padding: 2px 0;
    letter-spacing: var(--letter-spacing);
    font-size: var(--font-size);
    line-height: 1.6;
}

/* ===== 混合运算 ===== */
.mix-grid {
    display: grid;
    row-gap: var(--gap-mixed);
    padding: 0;
}
.mix-grid .preview-problem {
    letter-spacing: var(--letter-spacing);
    font-size: var(--font-size);
    line-height: var(--line-height-mixed);
}

/* ===== 页面内嵌二维码 ===== */
.page-qr {
    position: absolute;
    top: 4px;
    right: 6px;
}
.page-qr img {
    width: 30px;
    height: 30px;
    object-fit: contain;
}

.status {
    margin-top: 12px;
    padding: 10px 14px;
    border-radius: 8px;
    font-size: 0.85rem;
    display: none;
}
.status.success { display: block; background: rgba(76,175,80,0.15); color: #81c784; border: 1px solid rgba(76,175,80,0.3); }
.status.error { display: block; background: rgba(244,67,54,0.15); color: #e57373; border: 1px solid rgba(244,67,54,0.3); }
.status.info { display: block; background: rgba(79,138,139,0.15); color: #80cbc4; border: 1px solid rgba(79,138,139,0.3); }

@media (max-width: 600px) {
    .form-row { grid-template-columns: 1fr; }
    .btn-row { flex-direction: column; }
    .btn { width: 100%; }
}

/* ===== 打印样式：隐藏 UI，只打印预览区 ===== */
@media print {
    body { background: #fff !important; color: #000 !important; padding: 0; }
    .container { max-width: none; }
    .card, .header, .back-link, h2, .status { display: none !important; }
    .preview {
        display: block !important;
        background: #fff !important;
        color: #000 !important;
        padding: 0 !important;
        border-radius: 0 !important;
    }
    .preview-page {
        page-break-after: always;
        margin-bottom: 0;
        padding: 20px 40px;
    }
    .preview-page:last-child { page-break-after: avoid; }
}