/* ===== CSS Variables & Reset ===== */
:root {
    --bg-primary: #f0f4f8;
    --bg-secondary: #ffffff;
    --bg-tertiary: #e8eef4;
    --text-primary: #1a1a2e;
    --text-secondary: #5a6270;
    --border-color: #d0d8e0;
    --shadow: 0 4px 20px rgba(0, 0, 0, 0.08);

    /* Status Colors */
    --balanced-color: #2ecc71;
    --unbalanced-color: #e74c3c;
    --accent-color: #3498db;

    --radius: 12px;
    --transition: all 0.3s ease;
}

[data-theme="dark"] {
    --bg-primary: #0f0f1a;
    --bg-secondary: #1a1a2e;
    --bg-tertiary: #252540;
    --text-primary: #f0f4f8;
    --text-secondary: #a0a8b8;
    --border-color: #2a2a45;
    --shadow: 0 4px 20px rgba(0, 0, 0, 0.3);
}

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

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

/* ===== App Container ===== */
.app-container {
    max-width: 1800px;
    margin: 0 auto;
    padding: 15px;
    min-height: 100vh;
    display: flex;
    flex-direction: column;
}

/* ===== Header ===== */
.header {
    display: flex;
    justify-content: space-between;
    align-items: center;
    padding: 15px 25px;
    background: var(--bg-secondary);
    border-radius: var(--radius);
    margin-bottom: 15px;
    box-shadow: var(--shadow);
    flex-wrap: wrap;
    gap: 10px;
}

.header h1 {
    font-size: 1.5rem;
    font-weight: 700;
    background: linear-gradient(135deg, #4f883d, #6ab04c);
    -webkit-background-clip: text;
    -webkit-text-fill-color: transparent;
    background-clip: text;
}

.header-controls {
    display: flex;
    gap: 10px;
    align-items: center;
}

.action-btn {
    padding: 10px 20px;
    border: none;
    border-radius: 25px;
    font-size: 0.9rem;
    font-weight: 600;
    cursor: pointer;
    transition: var(--transition);
    background: linear-gradient(135deg, var(--accent-color), #2980b9);
    color: white;
}

.action-btn:hover {
    transform: scale(1.05);
    box-shadow: 0 4px 15px rgba(52, 152, 219, 0.4);
}

.action-btn.secondary {
    background: var(--bg-tertiary);
    color: var(--text-primary);
}

.action-btn.secondary:hover {
    background: var(--border-color);
    box-shadow: none;
}

.control-btn {
    width: 42px;
    height: 42px;
    border: 2px solid var(--border-color);
    border-radius: 50%;
    background: var(--bg-primary);
    cursor: pointer;
    display: flex;
    align-items: center;
    justify-content: center;
    transition: var(--transition);
}

.control-btn:hover {
    border-color: var(--accent-color);
    transform: scale(1.05);
}

.control-btn svg {
    width: 20px;
    height: 20px;
    color: var(--text-primary);
}

.moon-icon {
    display: none;
}

[data-theme="dark"] .sun-icon {
    display: none;
}

[data-theme="dark"] .moon-icon {
    display: block;
}

.compress-icon {
    display: none;
}

.fullscreen-active .expand-icon {
    display: none;
}

.fullscreen-active .compress-icon {
    display: block;
}

/* ===== Main Content ===== */
.main-content {
    display: grid;
    grid-template-columns: 280px minmax(0, 1fr);
    gap: 15px;
    flex: 1;
    min-width: 0;
}

.main-content > * {
    min-width: 0;
}

/* ===== Controls Panel ===== */
.controls-panel {
    background: var(--bg-secondary);
    border-radius: var(--radius);
    padding: 20px;
    box-shadow: var(--shadow);
    overflow-y: auto;
}

.controls-panel h2 {
    font-size: 1rem;
    font-weight: 600;
    margin-bottom: 15px;
    color: var(--text-secondary);
    text-transform: uppercase;
    letter-spacing: 0.5px;
}

/* Difficulty Selector */
.difficulty-selector {
    display: flex;
    gap: 8px;
    margin-bottom: 25px;
}

.difficulty-btn {
    flex: 1;
    padding: 10px 8px;
    border: 2px solid var(--border-color);
    border-radius: 8px;
    background: var(--bg-tertiary);
    font-size: 0.75rem;
    font-weight: 600;
    color: var(--text-secondary);
    cursor: pointer;
    transition: var(--transition);
}

.difficulty-btn:hover {
    border-color: var(--accent-color);
}

.difficulty-btn.active {
    background: var(--accent-color);
    border-color: var(--accent-color);
    color: white;
}

/* Examples Section */
.examples-section {
    margin-bottom: 20px;
}

.examples-section h3,
.stats-section h3,
.legend-section h3 {
    font-size: 0.8rem;
    font-weight: 600;
    color: var(--text-secondary);
    margin-bottom: 12px;
    text-transform: uppercase;
}

.examples-list {
    display: flex;
    flex-direction: column;
    gap: 8px;
    max-height: 440px;
    overflow-y: auto;
}

.equation-option {
    padding: 10px 12px;
    background: var(--bg-tertiary);
    border: 2px solid transparent;
    border-radius: 8px;
    font-size: 0.85rem;
    font-weight: 500;
    color: var(--text-primary);
    cursor: pointer;
    transition: var(--transition);
    text-align: left;
}

.equation-option:hover {
    border-color: var(--accent-color);
}

.equation-option.active {
    border-color: var(--accent-color);
    background: rgba(52, 152, 219, 0.1);
}

/* Stats */
.stats-section {
    margin-bottom: 20px;
    padding-top: 15px;
    border-top: 2px solid var(--border-color);
}

.stats-grid {
    display: grid;
    grid-template-columns: 1fr 1fr;
    gap: 10px;
}

.stat-item {
    background: var(--bg-tertiary);
    padding: 15px;
    border-radius: 10px;
    text-align: center;
}

.stat-value {
    display: block;
    font-size: 1.5rem;
    font-weight: 700;
    color: var(--accent-color);
}

.stat-label {
    font-size: 0.7rem;
    color: var(--text-secondary);
    text-transform: uppercase;
}

/* Legend */
.legend-section {
    padding-top: 15px;
    border-top: 2px solid var(--border-color);
}

.legend-items {
    display: flex;
    flex-direction: column;
    gap: 8px;
}

.legend-row {
    display: flex;
    align-items: center;
    gap: 10px;
    font-size: 0.75rem;
    color: var(--text-secondary);
}

.legend-dot {
    width: 14px;
    height: 14px;
    border-radius: 50%;
}

.legend-dot.balanced {
    background: var(--balanced-color);
}

.legend-dot.unbalanced {
    background: var(--unbalanced-color);
}

/* ===== Equation Container ===== */
.equation-container {
    background: var(--bg-secondary);
    border-radius: var(--radius);
    box-shadow: var(--shadow);
    padding: 30px;
    display: flex;
    flex-direction: column;
    position: relative;
}

.equation-container .info-panel {
    margin-top: 20px;
}

/* Equation Display */
.equation-display {
    display: flex;
    justify-content: center;
    align-items: center;
    gap: 20px;
    padding: 40px 20px;
    flex-wrap: wrap;
}

.equation-side {
    display: flex;
    align-items: center;
    gap: 15px;
    flex-wrap: wrap;
    justify-content: center;
}

/* Compound Box */
.compound {
    display: flex;
    align-items: center;
    gap: 8px;
    background: var(--bg-tertiary);
    padding: 15px 20px;
    border-radius: 12px;
    transition: var(--transition);
}

.compound:hover {
    box-shadow: 0 4px 15px rgba(0, 0, 0, 0.1);
}

.coefficient-controls {
    display: flex;
    flex-direction: column;
    gap: 4px;
}

.coef-btn {
    width: 28px;
    height: 28px;
    border: none;
    border-radius: 6px;
    background: var(--accent-color);
    color: white;
    font-size: 1rem;
    font-weight: 700;
    cursor: pointer;
    transition: var(--transition);
    display: flex;
    align-items: center;
    justify-content: center;
}

.coef-btn:hover {
    transform: scale(1.1);
    background: #2980b9;
}

.coef-btn:active {
    transform: scale(0.95);
}

.coefficient {
    font-size: 2rem;
    font-weight: 800;
    color: var(--accent-color);
    min-width: 30px;
    text-align: center;
}

.formula {
    font-size: 1.8rem;
    font-weight: 700;
}

.formula sub {
    font-size: 1rem;
    vertical-align: baseline;
    position: relative;
    top: 0.3em;
}

.plus-sign {
    font-size: 1.5rem;
    font-weight: 600;
    color: var(--text-secondary);
}

/* Arrow */
.equation-arrow {
    padding: 0 20px;
}

.arrow-symbol {
    font-size: 2.5rem;
    color: var(--text-secondary);
    animation: arrowPulse 2s ease-in-out infinite;
}

@keyframes arrowPulse {

    0%,
    100% {
        opacity: 1;
        transform: translateX(0);
    }

    50% {
        opacity: 0.6;
        transform: translateX(5px);
    }
}

/* Atom Counter */
.atom-counter {
    background: var(--bg-tertiary);
    border-radius: 12px;
    padding: 20px;
    margin-top: 20px;
}

.atom-counter h3 {
    font-size: 0.9rem;
    font-weight: 600;
    color: var(--text-secondary);
    margin-bottom: 15px;
    text-align: center;
    text-transform: uppercase;
}

.counter-grid {
    display: flex;
    justify-content: center;
    gap: 15px;
    flex-wrap: wrap;
}

.atom-count {
    background: var(--bg-secondary);
    border-radius: 10px;
    padding: 12px 20px;
    text-align: center;
    min-width: 80px;
    transition: var(--transition);
}

.atom-count.balanced {
    border: 2px solid var(--balanced-color);
    box-shadow: 0 0 10px rgba(46, 204, 113, 0.3);
}

.atom-count.unbalanced {
    border: 2px solid var(--unbalanced-color);
    box-shadow: 0 0 10px rgba(231, 76, 60, 0.3);
}

.atom-symbol {
    font-size: 1.2rem;
    font-weight: 700;
    margin-bottom: 5px;
}

.atom-values {
    display: flex;
    justify-content: center;
    gap: 10px;
    font-size: 0.9rem;
}

.atom-left,
.atom-right {
    font-weight: 600;
}

.atom-left {
    color: var(--accent-color);
}

.atom-right {
    color: var(--unbalanced-color);
}

.atom-count.balanced .atom-right {
    color: var(--balanced-color);
}

/* Status Message */
.status-message {
    display: flex;
    align-items: center;
    justify-content: center;
    gap: 10px;
    padding: 15px 25px;
    margin-top: 20px;
    background: linear-gradient(135deg, rgba(231, 76, 60, 0.1), rgba(241, 196, 15, 0.1));
    border: 2px solid var(--unbalanced-color);
    border-radius: 12px;
    font-size: 0.95rem;
    font-weight: 500;
    color: var(--text-primary);
}

.status-message.balanced {
    background: linear-gradient(135deg, rgba(46, 204, 113, 0.1), rgba(39, 174, 96, 0.1));
    border-color: var(--balanced-color);
}

.status-icon {
    font-size: 1.3rem;
}

/* Success Overlay */
.success-overlay {
    position: absolute;
    top: 0;
    left: 0;
    right: 0;
    bottom: 0;
    background: rgba(46, 204, 113, 0.95);
    border-radius: var(--radius);
    display: flex;
    align-items: center;
    justify-content: center;
    z-index: 100;
    animation: fadeIn 0.3s ease;
}

@keyframes fadeIn {
    from {
        opacity: 0;
    }

    to {
        opacity: 1;
    }
}

.success-content {
    text-align: center;
    color: white;
}

.success-icon {
    font-size: 4rem;
    display: block;
    margin-bottom: 15px;
}

.success-content h2 {
    font-size: 1.8rem;
    margin-bottom: 20px;
}

.next-btn {
    padding: 12px 30px;
    background: white;
    color: var(--balanced-color);
    border: none;
    border-radius: 25px;
    font-size: 1rem;
    font-weight: 600;
    cursor: pointer;
    transition: var(--transition);
}

.next-btn:hover {
    transform: scale(1.05);
    box-shadow: 0 4px 15px rgba(0, 0, 0, 0.2);
}

/* ===== Info Panel ===== */
.info-panel {
    background: var(--bg-secondary);
    border-radius: var(--radius);
    padding: 20px;
    box-shadow: var(--shadow);
    overflow-y: visible;
}

.info-panel h2 {
    font-size: 1rem;
    font-weight: 600;
    margin-bottom: 15px;
    color: var(--text-secondary);
    text-transform: uppercase;
}

.reaction-info {
    display: grid;
    grid-template-columns: repeat(2, minmax(0, 1fr));
    gap: 12px;
    margin-bottom: 20px;
}

.info-card {
    background: var(--bg-tertiary);
    padding: 15px;
    border-radius: 10px;
    min-width: 0;
}

.info-card.compact {
    grid-column: 1 / -1;
    display: flex;
    align-items: center;
    justify-content: space-between;
    gap: 12px;
    padding: 12px 16px;
}

.info-card h4 {
    font-size: 0.75rem;
    font-weight: 600;
    color: var(--text-secondary);
    text-transform: uppercase;
    margin-bottom: 8px;
}

.info-card p {
    font-size: 0.9rem;
    line-height: 1.5;
    color: var(--text-primary);
}

.info-card.compact h4 {
    margin-bottom: 0;
}

.info-card.compact p {
    margin: 0;
    font-size: 1rem;
    font-weight: 600;
}

/* Hint Section */
.hint-section {
    background: linear-gradient(135deg, rgba(241, 196, 15, 0.1), rgba(243, 156, 18, 0.1));
    border: 2px solid #f39c12;
    border-radius: 12px;
    padding: 15px;
    margin-bottom: 20px;
}

.hint-section h3 {
    font-size: 0.85rem;
    margin-bottom: 10px;
}

.hint-section p {
    font-size: 0.85rem;
    color: var(--text-secondary);
    margin-bottom: 10px;
    display: none;
}

.hint-section p.visible {
    display: block;
}

.hint-btn {
    padding: 8px 15px;
    background: #f39c12;
    color: white;
    border: none;
    border-radius: 20px;
    font-size: 0.8rem;
    font-weight: 600;
    cursor: pointer;
    transition: var(--transition);
}

.hint-btn:hover {
    background: #e67e22;
}

/* Explanation Box */
.explanation-box {
    background: var(--bg-tertiary);
    border-radius: 12px;
    padding: 15px;
}

.explanation-box h3 {
    font-size: 0.85rem;
    margin-bottom: 12px;
    color: var(--text-primary);
}

.explanation-box ol {
    padding-left: 20px;
}

.explanation-box li {
    font-size: 0.8rem;
    line-height: 1.8;
    color: var(--text-secondary);
}

/* ===== Responsive ===== */
@media (max-width: 1200px) {
    .main-content {
        grid-template-columns: 240px minmax(0, 1fr);
    }
}

@media (max-width: 1150px) and (min-width: 901px) {
    .app-container {
        padding: 10px;
    }

    .header {
        padding: 14px 18px;
        margin-bottom: 12px;
    }

    .main-content {
        grid-template-columns: 220px minmax(0, 1fr);
        gap: 12px;
    }

    .controls-panel,
    .info-panel {
        padding: 16px;
    }

    .equation-container {
        padding: 22px 18px;
    }

    .equation-display {
        gap: 12px;
        padding: 24px 8px;
        flex-wrap: nowrap;
    }

    .equation-side {
        gap: 10px;
        flex-wrap: nowrap;
    }

    .equation-arrow {
        padding: 0 8px;
    }

    .compound {
        gap: 6px;
        padding: 12px 14px;
    }

    .coefficient {
        font-size: 1.7rem;
        min-width: 24px;
    }

    .formula {
        font-size: 1.5rem;
    }

    .formula sub {
        font-size: 0.85rem;
    }

    .plus-sign {
        font-size: 1.3rem;
    }

    .arrow-symbol {
        font-size: 2.1rem;
    }

    .examples-list {
        max-height: 380px;
    }

    .equation-option {
        padding: 9px 10px;
        font-size: 0.8rem;
    }

    .info-card p,
    .hint-section p,
    .explanation-box li {
        font-size: 0.8rem;
    }

    .app-container[data-difficulty="hard"] .equation-display {
        gap: 8px;
    }

    .app-container[data-difficulty="hard"] .equation-side {
        gap: 8px;
    }

    .app-container[data-difficulty="hard"] .compound {
        gap: 5px;
        padding: 10px 12px;
    }

    .app-container[data-difficulty="hard"] .coefficient-controls {
        gap: 3px;
    }

    .app-container[data-difficulty="hard"] .coef-btn {
        width: 24px;
        height: 24px;
        border-radius: 5px;
        font-size: 0.9rem;
    }

    .app-container[data-difficulty="hard"] .coefficient {
        font-size: 1.35rem;
        min-width: 20px;
    }

    .app-container[data-difficulty="hard"] .formula {
        font-size: 1.22rem;
    }

    .app-container[data-difficulty="hard"] .formula sub {
        font-size: 0.72rem;
    }

    .app-container[data-difficulty="hard"] .plus-sign {
        font-size: 1.15rem;
    }

    .app-container[data-difficulty="hard"] .equation-arrow {
        padding: 0 6px;
    }

    .app-container[data-difficulty="hard"] .arrow-symbol {
        font-size: 1.8rem;
    }
}

@media (max-width: 900px) {
    .main-content {
        grid-template-columns: 1fr;
    }

    .controls-panel {
        order: 1;
    }

    .equation-container {
        order: 2;
    }

    .info-panel {
        order: 3;
    }

    .equation-display {
        flex-direction: column;
        gap: 15px;
    }

    .equation-arrow {
        transform: rotate(90deg);
    }

    .reaction-info {
        grid-template-columns: 1fr;
    }
}