:root {
    --primary: #6366f1;
    --primary-dark: #4f46e5;
    --secondary: #8b5cf6;
    --success: #10b981;
    --bg: #0f172a;
    --bg-card: #1e293b;
    --bg-elevated: #334155;
    --border: #475569;
    --text: #f1f5f9;
    --text-muted: #94a3b8;
    --text-dim: #64748b;
}

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

body {
    font-family: 'Inter', sans-serif;
    background: var(--bg);
    color: var(--text);
    padding: 40px 20px;
    min-height: 100vh;
}

.container { max-width: 860px; margin: 0 auto; }

.header {
    text-align: center;
    margin-bottom: 48px;
}

.header h1 {
    font-size: 36px;
    font-weight: 700;
    background: linear-gradient(135deg, var(--primary), var(--secondary));
    -webkit-background-clip: text;
    -webkit-text-fill-color: transparent;
    margin-bottom: 10px;
}

.header p {
    color: var(--text-muted);
    font-size: 15px;
}

.card {
    background: var(--bg-card);
    border: 1px solid var(--border);
    border-radius: 12px;
    padding: 32px;
    margin-bottom: 24px;
}

.input-label {
    font-size: 13px;
    font-weight: 500;
    color: var(--text-muted);
    text-transform: uppercase;
    letter-spacing: 0.6px;
    margin-bottom: 10px;
    display: block;
}

.reverse-grid {
    display: grid;
    grid-template-columns: 1fr 1fr;
    gap: 12px;
    margin-bottom: 12px;
}

.reverse-grid-3 {
    grid-template-columns: 1fr 1fr 1fr;
}

.reverse-label {
    font-size: 15px;
    color: var(--text-dim);
    margin-bottom: 8px;
    font-family: 'JetBrains Mono', monospace;
}

.hidden {
    display: none;
}

.hidden-native-select {
    position: absolute;
    width: 1px;
    height: 1px;
    opacity: 0;
    pointer-events: none;
    border: 0;
    padding: 0;
}

.math-select {
    position: relative;
}

.math-select-trigger {
    display: flex;
    align-items: center;
    justify-content: space-between;
    gap: 12px;
    text-align: left;
    cursor: pointer;
    min-height: 70px;
}

.math-select-main {
    min-width: 0;
    display: flex;
    flex-direction: column;
    gap: 3px;
}

.math-select-title {
    font-family: 'Inter', sans-serif;
    font-size: 17px;
    font-weight: 600;
    color: var(--text);
    white-space: nowrap;
    overflow: hidden;
    text-overflow: ellipsis;
}

.math-select-formula {
    font-family: 'Inter', sans-serif;
    font-size: 15px;
    color: var(--text-muted);
    white-space: nowrap;
    overflow: hidden;
    text-overflow: ellipsis;
}

.math-select-value {
    font-family: 'JetBrains Mono', monospace;
    font-size: 14px;
    color: var(--text-dim);
    flex: 0 0 auto;
}

.math-select-caret {
    color: var(--text-muted);
    font-size: 16px;
    line-height: 1;
    flex: 0 0 auto;
    transition: transform 0.2s ease;
}

.math-select.open .math-select-caret {
    transform: rotate(180deg);
}

.math-select-menu {
    position: absolute;
    top: calc(100% + 8px);
    left: 0;
    right: 0;
    z-index: 40;
    max-height: 480px;
    overflow: auto;
    background: var(--bg);
    border: 1px solid var(--border);
    border-radius: 10px;
    padding: 6px;
    box-shadow: 0 16px 34px rgba(2, 6, 23, 0.65);
}

.math-select-option {
    width: 100%;
    border: 1px solid transparent;
    border-radius: 8px;
    background: transparent;
    color: var(--text);
    padding: 12px 14px;
    text-align: left;
    cursor: pointer;
    display: grid;
    grid-template-columns: minmax(0, 1fr) auto;
    column-gap: 10px;
    row-gap: 4px;
}

.math-select-option:hover {
    background: rgba(99, 102, 241, 0.12);
    border-color: rgba(99, 102, 241, 0.35);
}

.math-select-option.active {
    background: rgba(99, 102, 241, 0.18);
    border-color: rgba(99, 102, 241, 0.55);
}

.math-select-option-title {
    font-family: 'Inter', sans-serif;
    font-size: 16px;
    font-weight: 600;
    color: var(--text);
    white-space: nowrap;
    overflow: hidden;
    text-overflow: ellipsis;
}

.math-select-option-formula {
    font-family: 'Inter', sans-serif;
    font-size: 15px;
    color: var(--text-muted);
    white-space: nowrap;
    overflow: hidden;
    text-overflow: ellipsis;
}

.math-select-option-value {
    grid-column: 2;
    grid-row: 1 / span 2;
    align-self: center;
    font-family: 'JetBrains Mono', monospace;
    font-size: 14px;
    color: var(--text-dim);
}

.math-select .katex,
.math-select-menu .katex {
    font-size: 1.1em;
}

.input-hint {
    font-size: 12px;
    color: var(--text-dim);
    font-family: 'JetBrains Mono', monospace;
    margin-top: 8px;
    line-height: 1.7;
}

.eq-input {
    width: 100%;
    padding: 14px 18px;
    background: var(--bg);
    border: 1px solid var(--border);
    border-radius: 8px;
    color: var(--text);
    font-family: 'JetBrains Mono', monospace;
    font-size: 16px;
    transition: border-color 0.2s;
}

.eq-input:focus {
    outline: none;
    border-color: var(--primary);
}

.btn-primary {
    margin-top: 20px;
    padding: 13px 32px;
    background: var(--primary);
    color: white;
    border: none;
    border-radius: 8px;
    font-weight: 600;
    font-size: 15px;
    cursor: pointer;
    transition: all 0.2s;
    font-family: 'Inter', sans-serif;
}

.btn-primary:hover {
    background: var(--primary-dark);
    transform: translateY(-1px);
}

.top-actions {
    margin-bottom: 16px;
}

.btn-secondary {
    display: inline-block;
    padding: 10px 16px;
    border: 1px solid var(--border);
    border-radius: 8px;
    background: var(--bg-card);
    color: var(--text);
    text-decoration: none;
    font-weight: 600;
    font-size: 14px;
    transition: all 0.2s;
}

.btn-secondary:hover {
    background: var(--bg-elevated);
    border-color: var(--primary);
}

.step-block {
    margin-bottom: 28px;
    animation: fadeIn 0.35s ease-out;
}

.step-label {
    font-size: 12px;
    font-weight: 700;
    text-transform: uppercase;
    letter-spacing: 1px;
    color: var(--primary);
    margin-bottom: 10px;
}

.math-box {
    background: var(--bg-elevated);
    border-left: 4px solid var(--primary);
    padding: 18px 22px;
    border-radius: 8px;
    overflow-x: auto;
    line-height: 2.2;
}

.math-box.result {
    border-left-color: var(--success);
    background: rgba(16, 185, 129, 0.07);
    padding: 22px 26px;
}

.prose {
    color: var(--text-muted);
    font-size: 14px;
    line-height: 1.8;
    margin-bottom: 12px;
}

.divider {
    border: none;
    border-top: 1px solid var(--border);
    margin: 24px 0;
}

.error-msg {
    background: rgba(239, 68, 68, 0.1);
    border: 1px solid rgba(239, 68, 68, 0.3);
    color: #ef4444;
    padding: 12px 16px;
    border-radius: 8px;
    font-size: 14px;
    margin-top: 16px;
}

@keyframes fadeIn {
    from { opacity: 0; transform: translateY(8px); }
    to   { opacity: 1; transform: translateY(0); }
}

#output { display: none; }

@media (max-width: 760px) {
    .reverse-grid {
        grid-template-columns: 1fr;
    }

    .reverse-grid-3 {
        grid-template-columns: 1fr;
    }
}