/* CSS Variables */
:root {
    --bg-primary: #0f1117;
    --bg-secondary: #1a1d24;
    --bg-card: #22262f;
    --bg-hover: #2a2f3a;
    --text-primary: #f0f2f5;
    --text-secondary: #a0a6b1;
    --text-muted: #6b7280;
    --accent-primary: #3b82f6;
    --accent-green: #22c55e;
    --accent-red: #ef4444;
    --radius-sm: 8px;
    --radius-md: 12px;
    --radius-lg: 16px;
    --transition-fast: 150ms ease;
}

* { box-sizing: border-box; margin: 0; padding: 0; }

body {
    font-family: 'Inter', -apple-system, BlinkMacSystemFont, sans-serif;
    background: var(--bg-primary);
    color: var(--text-primary);
    line-height: 1.6;
    min-height: 100vh;
}

.app {
    max-width: 600px;
    margin: 0 auto;
    padding: 24px;
}

.back-link {
    display: inline-block;
    color: var(--text-secondary);
    text-decoration: none;
    font-size: 0.9rem;
    margin-bottom: 24px;
    transition: var(--transition-fast);
}
.back-link:hover { color: var(--accent-primary); }

.header { text-align: center; margin-bottom: 32px; }
.title { font-size: 2rem; font-weight: 700; margin-bottom: 8px; }
.subtitle { color: var(--text-secondary); }

.calculator { margin-bottom: 32px; }

.calc-card {
    background: var(--bg-card);
    border-radius: var(--radius-lg);
    padding: 20px;
    margin-bottom: 16px;
}

.calc-title {
    font-size: 1rem;
    font-weight: 600;
    color: var(--text-secondary);
    margin-bottom: 16px;
}

.calc-row {
    display: flex;
    align-items: center;
    flex-wrap: wrap;
    gap: 10px;
    margin-bottom: 16px;
}

.calc-text {
    color: var(--text-muted);
    font-size: 0.95rem;
}

.calc-input {
    width: 100px;
    padding: 12px 14px;
    background: var(--bg-secondary);
    border: 1px solid var(--bg-hover);
    border-radius: var(--radius-sm);
    color: var(--text-primary);
    font-size: 1.1rem;
    text-align: center;
    transition: var(--transition-fast);
}
.calc-input.small { width: 70px; }
.calc-input:focus {
    outline: none;
    border-color: var(--accent-primary);
    box-shadow: 0 0 0 3px rgba(59, 130, 246, 0.2);
}

.calc-select {
    padding: 12px 14px;
    background: var(--bg-secondary);
    border: 1px solid var(--bg-hover);
    border-radius: var(--radius-sm);
    color: var(--text-primary);
    font-size: 0.95rem;
    cursor: pointer;
}
.calc-select:focus {
    outline: none;
    border-color: var(--accent-primary);
}

.calc-result {
    display: flex;
    justify-content: space-between;
    align-items: center;
    padding: 14px 16px;
    background: var(--bg-secondary);
    border-radius: var(--radius-md);
}
.result-label { color: var(--text-muted); }
.result-value {
    font-size: 1.3rem;
    font-weight: 700;
    color: var(--accent-green);
}
.result-value.negative { color: var(--accent-red); }

/* Formulas Section */
.formulas-section {
    margin-bottom: 32px;
}
.formulas-section h2 {
    font-size: 1.1rem;
    color: var(--text-secondary);
    margin-bottom: 16px;
}
.formula-grid {
    display: grid;
    grid-template-columns: repeat(2, 1fr);
    gap: 12px;
}
.formula-card {
    background: var(--bg-card);
    padding: 16px;
    border-radius: var(--radius-md);
    text-align: center;
}
.formula-card h4 {
    font-size: 0.85rem;
    color: var(--text-muted);
    margin-bottom: 8px;
    font-weight: 500;
}
.formula-card code {
    font-size: 0.9rem;
    color: var(--accent-primary);
    font-family: monospace;
}

/* FAQ */
.faq-section { margin-bottom: 32px; }
.faq-title { font-size: 1.25rem; margin-bottom: 16px; }
.faq-item {
    background: var(--bg-card);
    border-radius: var(--radius-md);
    margin-bottom: 8px;
    overflow: hidden;
}
.faq-question {
    width: 100%;
    padding: 16px;
    font-size: 1rem;
    font-weight: 600;
    color: var(--text-primary);
    cursor: pointer;
    list-style: none;
}
.faq-question::-webkit-details-marker { display: none; }
.faq-question:hover { background: var(--bg-hover); }
.faq-answer { padding: 0 16px 16px; color: var(--text-secondary); }
.faq-answer ul { margin-left: 20px; margin-top: 8px; }
.faq-answer li { margin-bottom: 4px; }

/* Footer */
.footer { text-align: center; color: var(--text-muted); font-size: 0.85rem; }
.related-tools { margin-top: 8px; }
.related-tools a { color: var(--accent-primary); text-decoration: none; }
.related-tools a:hover { text-decoration: underline; }

@media (max-width: 500px) {
    .calc-row { gap: 8px; }
    .calc-input { width: 80px; padding: 10px; font-size: 1rem; }
    .calc-input.small { width: 60px; }
    .formula-grid { grid-template-columns: 1fr; }
}