/* ===== Formula Derivation Tool Styles ===== */
/* Complete styling with Light/Dark Theme Support & Responsive Design */

/* ===== CSS Variables ===== */
.formula-derivation-container {
    --fd-primary: var(--primary-color, #6366f1);
    --fd-primary-rgb: var(--primary-rgb, 99, 102, 241);
    --fd-secondary: var(--secondary-color, #06b6d4);
    --fd-secondary-rgb: 6, 182, 212;
    --fd-success: #10b981;
    --fd-warning: #f59e0b;
    --fd-danger: #ef4444;
    --fd-info: #3b82f6;
    --fd-purple: #8b5cf6;
    --fd-orange: #f97316;
    --fd-teal: #14b8a6;
    --fd-gradient: linear-gradient(135deg, var(--fd-primary), var(--fd-secondary));

    --fd-bg: var(--card-bg-color, #ffffff);
    --fd-surface: var(--background-color, #f9fafb);
    --fd-text: var(--text-color, #1f2937);
    --fd-text-muted: #6b7280;
    --fd-border: var(--border-color, #e5e7eb);
    --fd-radius: 10px;
    --fd-radius-lg: 16px;
    --fd-shadow: 0 4px 6px -1px rgba(0, 0, 0, 0.1), 0 2px 4px -2px rgba(0, 0, 0, 0.1);
    --fd-shadow-lg: 0 10px 15px -3px rgba(0, 0, 0, 0.1), 0 4px 6px -4px rgba(0, 0, 0, 0.1);
    --fd-transition: 0.2s ease;

    font-family: var(--font-family, 'Inter', -apple-system, BlinkMacSystemFont, sans-serif);
    color: var(--fd-text);
    max-width: 1600px;
    margin: 0 auto;
    padding: 0 1rem;
}

/* Dark Theme Variables */
.dark-theme .formula-derivation-container,
[data-theme="dark"] .formula-derivation-container {
    --fd-text-muted: #9ca3af;
    --fd-surface: #1f2937;
    --fd-bg: #111827;
    --fd-border: #374151;
}

/* ===== Header ===== */
.fd-header {
    display: flex;
    justify-content: space-between;
    align-items: center;
    flex-wrap: wrap;
    gap: 1.5rem;
    padding: 1.75rem;
    background: linear-gradient(135deg, rgba(var(--fd-primary-rgb), 0.08), rgba(var(--fd-secondary-rgb), 0.05));
    border-radius: var(--fd-radius-lg);
    margin-bottom: 1.5rem;
    border: 1px solid rgba(var(--fd-primary-rgb), 0.12);
}

.fd-header .header-content {
    display: flex;
    align-items: center;
    gap: 1.25rem;
}

.fd-header .header-icon {
    width: 60px;
    height: 60px;
    background: var(--fd-gradient);
    border-radius: 14px;
    display: flex;
    align-items: center;
    justify-content: center;
    color: white;
    font-size: 1.5rem;
    box-shadow: 0 8px 20px rgba(var(--fd-primary-rgb), 0.3);
}

.fd-header .header-text h2 {
    margin: 0;
    font-size: 1.4rem;
    font-weight: 800;
    color: var(--fd-text);
}

.fd-header .header-text p {
    margin: 0.35rem 0 0;
    font-size: 0.9rem;
    color: var(--fd-text-muted);
}

.header-badges {
    display: flex;
    gap: 0.6rem;
    flex-wrap: wrap;
}

.fd-badge {
    padding: 0.5rem 1rem;
    border-radius: 20px;
    font-size: 0.75rem;
    font-weight: 600;
    display: inline-flex;
    align-items: center;
    gap: 0.4rem;
}

.fd-badge.secure {
    background: rgba(16, 185, 129, 0.1);
    color: #10b981;
}

.fd-badge.academic {
    background: rgba(99, 102, 241, 0.1);
    color: var(--fd-primary);
}

.fd-badge.math {
    background: rgba(6, 182, 212, 0.1);
    color: #06b6d4;
}

.dark-theme .fd-badge.secure {
    background: rgba(16, 185, 129, 0.15);
}

.dark-theme .fd-badge.academic {
    background: rgba(99, 102, 241, 0.15);
}

.dark-theme .fd-badge.math {
    background: rgba(6, 182, 212, 0.15);
}

.header-actions {
    display: flex;
    gap: 0.5rem;
}

.formula-derivation-container .btn-icon {
    width: 44px;
    height: 44px;
    display: flex;
    align-items: center;
    justify-content: center;
    border: 1px solid var(--fd-border);
    border-radius: var(--fd-radius);
    background: var(--fd-bg);
    color: var(--fd-text);
    cursor: pointer;
    transition: var(--fd-transition);
}

.formula-derivation-container .btn-icon:hover {
    border-color: var(--fd-primary);
    color: var(--fd-primary);
    background: rgba(var(--fd-primary-rgb), 0.05);
    transform: translateY(-2px);
}

/* ===== Notice ===== */
.fd-notice {
    display: flex;
    align-items: flex-start;
    gap: 1rem;
    padding: 1rem 1.25rem;
    background: rgba(var(--fd-primary-rgb), 0.05);
    border: 1px solid rgba(var(--fd-primary-rgb), 0.12);
    border-radius: var(--fd-radius);
    margin-bottom: 1.5rem;
}

.fd-notice>i {
    color: var(--fd-primary);
    font-size: 1.25rem;
    flex-shrink: 0;
    margin-top: 0.1rem;
}

.fd-notice .notice-content {
    font-size: 0.9rem;
    line-height: 1.6;
    color: var(--fd-text);
}

.fd-notice strong {
    color: var(--fd-primary);
}

/* ===== Main Layout ===== */
.fd-layout {
    display: grid;
    grid-template-columns: 280px 1fr 280px;
    gap: 1.5rem;
    align-items: start;
}

/* ===== Sidebar ===== */
.fd-sidebar {
    background: var(--fd-bg);
    border: 1px solid var(--fd-border);
    border-radius: var(--fd-radius-lg);
    padding: 1.25rem;
    position: sticky;
    top: 100px;
}

.sidebar-section {
    margin-bottom: 1.5rem;
}

.sidebar-section:last-child {
    margin-bottom: 0;
}

.sidebar-section h3 {
    display: flex;
    align-items: center;
    gap: 0.5rem;
    margin: 0 0 0.75rem;
    font-size: 0.9rem;
    font-weight: 700;
    color: var(--fd-text);
}

.sidebar-section h3 i {
    color: var(--fd-primary);
    font-size: 0.85rem;
}

.fd-select {
    width: 100%;
    padding: 0.75rem 1rem;
    border: 2px solid var(--fd-border);
    border-radius: var(--fd-radius);
    background: var(--fd-surface);
    color: var(--fd-text);
    font-size: 0.9rem;
    font-family: inherit;
    cursor: pointer;
    transition: var(--fd-transition);
}

.fd-select:focus {
    outline: none;
    border-color: var(--fd-primary);
    box-shadow: 0 0 0 4px rgba(var(--fd-primary-rgb), 0.1);
}

/* Formula Search */
.formula-search {
    position: relative;
    margin-bottom: 0.75rem;
}

.formula-search i {
    position: absolute;
    left: 0.9rem;
    top: 50%;
    transform: translateY(-50%);
    color: var(--fd-text-muted);
    font-size: 0.85rem;
}

.formula-search input {
    width: 100%;
    padding: 0.65rem 0.9rem 0.65rem 2.5rem;
    border: 2px solid var(--fd-border);
    border-radius: var(--fd-radius);
    background: var(--fd-surface);
    color: var(--fd-text);
    font-size: 0.85rem;
    font-family: inherit;
    transition: var(--fd-transition);
}

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

/* Formula List */
.formula-list {
    max-height: 250px;
    overflow-y: auto;
    display: flex;
    flex-direction: column;
    gap: 0.35rem;
}

.formula-item {
    display: flex;
    align-items: center;
    justify-content: space-between;
    padding: 0.7rem 0.9rem;
    border: 1px solid transparent;
    border-radius: var(--fd-radius);
    background: var(--fd-surface);
    cursor: pointer;
    transition: var(--fd-transition);
}

.formula-item:hover {
    border-color: var(--fd-primary);
    background: rgba(var(--fd-primary-rgb), 0.05);
}

.formula-item.active {
    background: var(--fd-gradient);
    color: white;
    border-color: transparent;
}

.formula-item .formula-label {
    font-size: 0.85rem;
    font-weight: 500;
}

.formula-item .formula-eq {
    font-size: 0.75rem;
    font-family: 'Cambria Math', 'Times New Roman', serif;
    opacity: 0.8;
}

.formula-item.active .formula-eq {
    opacity: 1;
}

/* Custom Equation Input */
.custom-equation-input {
    display: flex;
    gap: 0.5rem;
}

.custom-equation-input input {
    flex: 1;
    padding: 0.7rem 1rem;
    border: 2px solid var(--fd-border);
    border-radius: var(--fd-radius);
    background: var(--fd-surface);
    color: var(--fd-text);
    font-family: 'Cambria Math', 'Times New Roman', serif;
    font-size: 0.95rem;
    transition: var(--fd-transition);
}

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

.btn-parse {
    width: 42px;
    height: 42px;
    display: flex;
    align-items: center;
    justify-content: center;
    border: none;
    border-radius: var(--fd-radius);
    background: var(--fd-gradient);
    color: white;
    cursor: pointer;
    transition: var(--fd-transition);
}

.btn-parse:hover {
    transform: scale(1.05);
    box-shadow: 0 4px 12px rgba(var(--fd-primary-rgb), 0.3);
}

.custom-help {
    margin-top: 0.5rem;
}

.custom-help small {
    color: var(--fd-text-muted);
    font-size: 0.75rem;
}

/* History List */
.history-list {
    max-height: 150px;
    overflow-y: auto;
    display: flex;
    flex-direction: column;
    gap: 0.35rem;
}

.history-item {
    display: flex;
    align-items: center;
    justify-content: space-between;
    padding: 0.6rem 0.75rem;
    background: var(--fd-surface);
    border-radius: var(--fd-radius);
    cursor: pointer;
    transition: var(--fd-transition);
    font-size: 0.8rem;
}

.history-item:hover {
    background: rgba(var(--fd-primary-rgb), 0.08);
}

.history-item .h-name {
    font-weight: 500;
    color: var(--fd-text);
}

.history-item .h-time {
    font-size: 0.7rem;
    color: var(--fd-text-muted);
}

.history-empty {
    text-align: center;
    padding: 0.75rem;
    color: var(--fd-text-muted);
    font-size: 0.8rem;
}

/* ===== Main Content ===== */
.fd-main {
    background: var(--fd-bg);
    border: 1px solid var(--fd-border);
    border-radius: var(--fd-radius-lg);
    padding: 1.5rem;
    min-height: 600px;
}

/* Mode Toggle */
.fd-mode-toggle {
    display: flex;
    flex-wrap: wrap;
    gap: 0.5rem;
    padding: 0.5rem;
    background: var(--fd-surface);
    border-radius: var(--fd-radius);
    margin-bottom: 1rem;
}

.mode-btn {
    display: flex;
    align-items: center;
    gap: 0.5rem;
    padding: 0.7rem 1.25rem;
    border: none;
    border-radius: var(--fd-radius);
    background: transparent;
    color: var(--fd-text-muted);
    font-size: 0.9rem;
    font-weight: 600;
    cursor: pointer;
    transition: var(--fd-transition);
}

.mode-btn:hover {
    background: var(--fd-bg);
    color: var(--fd-text);
}

.mode-btn.active {
    background: var(--fd-gradient);
    color: white;
    box-shadow: 0 4px 12px rgba(var(--fd-primary-rgb), 0.25);
}

/* Student Mode Toggle */
.fd-student-mode {
    display: flex;
    justify-content: space-between;
    align-items: center;
    flex-wrap: wrap;
    gap: 1rem;
    padding: 0.75rem 1rem;
    background: var(--fd-surface);
    border-radius: var(--fd-radius);
    margin-bottom: 1.5rem;
}

.mode-switch {
    display: flex;
    gap: 0.35rem;
    padding: 0.25rem;
    background: var(--fd-bg);
    border-radius: var(--fd-radius);
    border: 1px solid var(--fd-border);
}

.student-btn {
    display: flex;
    align-items: center;
    gap: 0.4rem;
    padding: 0.5rem 1rem;
    border: none;
    border-radius: 6px;
    background: transparent;
    color: var(--fd-text-muted);
    font-size: 0.8rem;
    font-weight: 600;
    cursor: pointer;
    transition: var(--fd-transition);
}

.student-btn:hover {
    color: var(--fd-text);
}

.student-btn.active {
    background: var(--fd-gradient);
    color: white;
}

.mode-options {
    display: flex;
    gap: 1.25rem;
    flex-wrap: wrap;
}

.toggle-option {
    display: flex;
    align-items: center;
    gap: 0.5rem;
    font-size: 0.8rem;
    color: var(--fd-text-muted);
    cursor: pointer;
}

.toggle-option input[type="checkbox"] {
    width: 16px;
    height: 16px;
    accent-color: var(--fd-primary);
    cursor: pointer;
}

/* Panels */
.fd-panel {
    display: none;
    animation: fadeIn 0.3s ease;
}

.fd-panel.active {
    display: block;
}

@keyframes fadeIn {
    from {
        opacity: 0;
        transform: translateY(10px);
    }

    to {
        opacity: 1;
        transform: translateY(0);
    }
}

.panel-header {
    display: flex;
    justify-content: space-between;
    align-items: center;
    flex-wrap: wrap;
    gap: 1rem;
    margin-bottom: 1.5rem;
    padding-bottom: 1rem;
    border-bottom: 1px solid var(--fd-border);
}

.panel-header h3 {
    display: flex;
    align-items: center;
    gap: 0.5rem;
    margin: 0;
    font-size: 1.1rem;
    color: var(--fd-text);
}

.panel-header h3 i {
    color: var(--fd-primary);
}

/* Selected Formula Card */
.selected-formula-card {
    background: linear-gradient(135deg, rgba(var(--fd-primary-rgb), 0.08), rgba(var(--fd-secondary-rgb), 0.05));
    border: 1px solid rgba(var(--fd-primary-rgb), 0.15);
    border-radius: var(--fd-radius);
    padding: 1.25rem;
    margin-bottom: 1.5rem;
}

.formula-display {
    display: flex;
    align-items: baseline;
    gap: 1rem;
    flex-wrap: wrap;
    margin-bottom: 0.75rem;
}

.formula-name {
    font-size: 1rem;
    font-weight: 700;
    color: var(--fd-text);
}

.formula-equation {
    font-size: 1.5rem;
    font-family: 'Cambria Math', 'Times New Roman', serif;
    color: var(--fd-primary);
    font-weight: 500;
}

.formula-info {
    display: flex;
    gap: 0.75rem;
    flex-wrap: wrap;
}

.info-badge {
    display: inline-flex;
    align-items: center;
    gap: 0.35rem;
    padding: 0.35rem 0.75rem;
    background: var(--fd-bg);
    border-radius: 20px;
    font-size: 0.75rem;
    color: var(--fd-text-muted);
}

.info-badge i {
    color: var(--fd-primary);
    font-size: 0.7rem;
}

/* Derivation Container */
.derivation-container {
    min-height: 300px;
}

.empty-state {
    display: flex;
    flex-direction: column;
    align-items: center;
    justify-content: center;
    padding: 3rem;
    text-align: center;
    color: var(--fd-text-muted);
}

.empty-state i {
    font-size: 3rem;
    margin-bottom: 1rem;
    opacity: 0.4;
}

.empty-state h4 {
    margin: 0 0 0.5rem;
    font-size: 1.1rem;
    color: var(--fd-text);
}

.empty-state p {
    margin: 0;
    font-size: 0.9rem;
    max-width: 300px;
}

/* Derivation Steps */
.derivation-steps {
    display: flex;
    flex-direction: column;
    gap: 1rem;
}

.derivation-step {
    display: flex;
    gap: 1rem;
    padding: 1.25rem;
    background: var(--fd-surface);
    border: 1px solid var(--fd-border);
    border-radius: var(--fd-radius);
    transition: var(--fd-transition);
    animation: stepFadeIn 0.4s ease forwards;
    opacity: 0;
    transform: translateX(-20px);
}

@keyframes stepFadeIn {
    to {
        opacity: 1;
        transform: translateX(0);
    }
}

.derivation-step:hover {
    border-color: var(--fd-primary);
    box-shadow: 0 4px 12px rgba(0, 0, 0, 0.06);
}

.dark-theme .derivation-step:hover {
    box-shadow: 0 4px 12px rgba(0, 0, 0, 0.3);
}

.derivation-step.highlight {
    border-left: 4px solid var(--fd-primary);
    background: rgba(var(--fd-primary-rgb), 0.05);
}

.step-number {
    width: 36px;
    height: 36px;
    display: flex;
    align-items: center;
    justify-content: center;
    background: var(--fd-gradient);
    color: white;
    border-radius: 50%;
    font-weight: 700;
    font-size: 0.9rem;
    flex-shrink: 0;
}

.step-content {
    flex: 1;
}

.step-equation {
    font-size: 1.25rem;
    font-family: 'Cambria Math', 'Times New Roman', serif;
    color: var(--fd-text);
    margin-bottom: 0.5rem;
    padding: 0.75rem;
    background: rgba(var(--fd-primary-rgb), 0.05);
    border-radius: var(--fd-radius);
    display: inline-block;
}

.step-explanation {
    font-size: 0.9rem;
    color: var(--fd-text-muted);
    line-height: 1.6;
    margin-top: 0.5rem;
}

.step-explanation strong {
    color: var(--fd-primary);
}

.step-law {
    display: inline-flex;
    align-items: center;
    gap: 0.35rem;
    padding: 0.35rem 0.75rem;
    background: rgba(139, 92, 246, 0.1);
    color: var(--fd-purple);
    border-radius: 20px;
    font-size: 0.75rem;
    margin-top: 0.75rem;
}

.dark-theme .step-law {
    background: rgba(139, 92, 246, 0.15);
}

/* QED Box */
.qed-box {
    display: flex;
    align-items: center;
    justify-content: center;
    gap: 1rem;
    padding: 1.25rem;
    background: linear-gradient(135deg, rgba(16, 185, 129, 0.1), rgba(6, 182, 212, 0.08));
    border: 1px solid rgba(16, 185, 129, 0.2);
    border-radius: var(--fd-radius);
    margin-top: 1rem;
    flex-wrap: wrap;
}

.qed-box i {
    color: var(--fd-success);
    font-size: 1.5rem;
}

.qed-text {
    font-size: 1.1rem;
    font-weight: 700;
    color: var(--fd-success);
}

.qed-result {
    font-size: 1.35rem;
    font-family: 'Cambria Math', 'Times New Roman', serif;
    color: var(--fd-text);
}

/* Diagram Section */
.diagram-section,
.laws-section {
    margin-top: 2rem;
    padding-top: 1.5rem;
    border-top: 1px solid var(--fd-border);
}

.diagram-section h4,
.laws-section h4 {
    display: flex;
    align-items: center;
    gap: 0.5rem;
    margin: 0 0 1rem;
    font-size: 1rem;
    color: var(--fd-text);
}

.diagram-section h4 i,
.laws-section h4 i {
    color: var(--fd-primary);
}

.diagram-container {
    display: flex;
    justify-content: center;
    padding: 1.5rem;
    background: var(--fd-surface);
    border-radius: var(--fd-radius);
    border: 1px solid var(--fd-border);
}

.diagram-container svg {
    max-width: 100%;
    height: auto;
}

.laws-container {
    display: flex;
    flex-wrap: wrap;
    gap: 0.75rem;
}

.law-badge {
    display: inline-flex;
    align-items: center;
    gap: 0.5rem;
    padding: 0.6rem 1rem;
    background: rgba(var(--fd-primary-rgb), 0.08);
    border: 1px solid rgba(var(--fd-primary-rgb), 0.15);
    border-radius: 25px;
    font-size: 0.85rem;
    color: var(--fd-text);
}

.law-badge i {
    color: var(--fd-purple);
}

/* ===== Given-To-Prove & Rearrange Panels ===== */
.prove-inputs,
.rearrange-inputs {
    display: flex;
    flex-direction: column;
    gap: 1rem;
    padding: 1.25rem;
    background: var(--fd-surface);
    border-radius: var(--fd-radius);
    margin-bottom: 1.5rem;
}

.prove-section,
.rearrange-section {
    display: flex;
    flex-direction: column;
    gap: 0.5rem;
}

.prove-section label,
.rearrange-section label {
    display: flex;
    align-items: center;
    gap: 0.5rem;
    font-size: 0.9rem;
    font-weight: 600;
    color: var(--fd-text);
}

.prove-section label i,
.rearrange-section label i {
    color: var(--fd-primary);
}

.prove-section input,
.rearrange-section input {
    padding: 0.85rem 1rem;
    border: 2px solid var(--fd-border);
    border-radius: var(--fd-radius);
    background: var(--fd-bg);
    color: var(--fd-text);
    font-family: 'Cambria Math', 'Times New Roman', serif;
    font-size: 1rem;
    transition: var(--fd-transition);
}

.prove-section input:focus,
.rearrange-section input:focus {
    outline: none;
    border-color: var(--fd-primary);
    box-shadow: 0 0 0 4px rgba(var(--fd-primary-rgb), 0.1);
}

.btn-primary.fd-btn {
    display: inline-flex;
    align-items: center;
    justify-content: center;
    gap: 0.5rem;
    padding: 0.9rem 1.75rem;
    border: none;
    border-radius: var(--fd-radius);
    background: var(--fd-gradient);
    color: white;
    font-size: 1rem;
    font-weight: 600;
    cursor: pointer;
    transition: var(--fd-transition);
    margin-top: 0.5rem;
    width: fit-content;
}

.btn-primary.fd-btn:hover {
    transform: translateY(-2px);
    box-shadow: 0 6px 20px rgba(var(--fd-primary-rgb), 0.35);
}

.prove-result,
.rearrange-result {
    min-height: 250px;
}

/* Variable Selector */
.variable-selector {
    display: flex;
    flex-wrap: wrap;
    gap: 0.5rem;
    margin-bottom: 0.75rem;
}

.variable-btn {
    padding: 0.5rem 1rem;
    border: 2px solid var(--fd-border);
    border-radius: var(--fd-radius);
    background: var(--fd-bg);
    color: var(--fd-text);
    font-family: 'Cambria Math', 'Times New Roman', serif;
    font-size: 1rem;
    cursor: pointer;
    transition: var(--fd-transition);
}

.variable-btn:hover {
    border-color: var(--fd-primary);
    color: var(--fd-primary);
}

.variable-btn.active {
    background: var(--fd-gradient);
    border-color: transparent;
    color: white;
}

/* ===== Explorer Panel ===== */
.explorer-filters {
    display: flex;
    flex-wrap: wrap;
    gap: 1rem;
    padding: 1rem;
    background: var(--fd-surface);
    border-radius: var(--fd-radius);
    margin-bottom: 1rem;
}

.filter-group {
    display: flex;
    align-items: center;
    gap: 0.5rem;
}

.filter-group label {
    font-size: 0.85rem;
    color: var(--fd-text-muted);
    white-space: nowrap;
}

.filter-group select {
    padding: 0.6rem 1rem;
    border: 2px solid var(--fd-border);
    border-radius: var(--fd-radius);
    background: var(--fd-bg);
    color: var(--fd-text);
    font-size: 0.85rem;
    cursor: pointer;
}

.filter-group.search-group {
    flex: 1;
    min-width: 200px;
    position: relative;
}

.filter-group.search-group i {
    position: absolute;
    left: 0.9rem;
    top: 50%;
    transform: translateY(-50%);
    color: var(--fd-text-muted);
}

.filter-group.search-group input {
    width: 100%;
    padding: 0.6rem 1rem 0.6rem 2.5rem;
    border: 2px solid var(--fd-border);
    border-radius: var(--fd-radius);
    background: var(--fd-bg);
    color: var(--fd-text);
    font-size: 0.85rem;
}

.explorer-stats {
    display: flex;
    gap: 1.5rem;
    padding: 0.75rem 0;
    margin-bottom: 1rem;
}

.stat-item {
    display: flex;
    align-items: center;
    gap: 0.4rem;
    font-size: 0.85rem;
    color: var(--fd-text-muted);
}

.stat-item i {
    color: var(--fd-primary);
}

.explorer-grid {
    display: grid;
    grid-template-columns: repeat(auto-fill, minmax(260px, 1fr));
    gap: 1rem;
}

.explorer-card {
    padding: 1.25rem;
    background: var(--fd-surface);
    border: 1px solid var(--fd-border);
    border-radius: var(--fd-radius);
    cursor: pointer;
    transition: var(--fd-transition);
}

.explorer-card:hover {
    border-color: var(--fd-primary);
    box-shadow: var(--fd-shadow);
    transform: translateY(-3px);
}

.explorer-card .card-subject {
    display: inline-flex;
    align-items: center;
    gap: 0.35rem;
    padding: 0.3rem 0.7rem;
    background: rgba(var(--fd-primary-rgb), 0.1);
    color: var(--fd-primary);
    border-radius: 20px;
    font-size: 0.7rem;
    font-weight: 600;
    margin-bottom: 0.75rem;
}

.explorer-card .card-name {
    font-size: 1rem;
    font-weight: 700;
    color: var(--fd-text);
    margin-bottom: 0.35rem;
}

.explorer-card .card-equation {
    font-family: 'Cambria Math', 'Times New Roman', serif;
    font-size: 1.15rem;
    color: var(--fd-primary);
    margin-bottom: 0.75rem;
}

.explorer-card .card-meta {
    display: flex;
    gap: 0.75rem;
    flex-wrap: wrap;
}

.explorer-card .meta-item {
    display: flex;
    align-items: center;
    gap: 0.3rem;
    font-size: 0.75rem;
    color: var(--fd-text-muted);
}

.explorer-card .meta-item i {
    color: var(--fd-primary);
    font-size: 0.7rem;
}

/* Difficulty Badges */
.difficulty-badge {
    padding: 0.2rem 0.5rem;
    border-radius: 10px;
    font-size: 0.65rem;
    font-weight: 600;
    text-transform: uppercase;
}

.difficulty-badge.easy {
    background: rgba(16, 185, 129, 0.1);
    color: #10b981;
}

.difficulty-badge.medium {
    background: rgba(245, 158, 11, 0.1);
    color: #f59e0b;
}

.difficulty-badge.hard {
    background: rgba(239, 68, 68, 0.1);
    color: #ef4444;
}

/* ===== Quick Reference Sidebar ===== */
.fd-quick-ref {
    display: flex;
    flex-direction: column;
    gap: 1rem;
    position: sticky;
    top: 100px;
}

.quick-ref-card {
    background: var(--fd-bg);
    border: 1px solid var(--fd-border);
    border-radius: var(--fd-radius);
    padding: 1rem;
}

.quick-ref-card h4 {
    display: flex;
    align-items: center;
    gap: 0.5rem;
    margin: 0 0 0.75rem;
    font-size: 0.9rem;
    color: var(--fd-text);
    padding-bottom: 0.75rem;
    border-bottom: 1px solid var(--fd-border);
}

.quick-ref-card h4 i {
    color: var(--fd-primary);
}

.notes-content,
.related-list {
    display: flex;
    flex-direction: column;
    gap: 0.5rem;
    max-height: 180px;
    overflow-y: auto;
}

.note-item {
    font-size: 0.85rem;
    color: var(--fd-text);
    padding: 0.5rem;
    background: var(--fd-surface);
    border-radius: var(--fd-radius);
}

.note-item.muted {
    color: var(--fd-text-muted);
    text-align: center;
}

.related-item {
    display: flex;
    justify-content: space-between;
    align-items: center;
    padding: 0.5rem 0.75rem;
    background: var(--fd-surface);
    border-radius: var(--fd-radius);
    cursor: pointer;
    transition: var(--fd-transition);
    font-size: 0.8rem;
}

.related-item:hover {
    background: rgba(var(--fd-primary-rgb), 0.08);
}

.reference-content {
    display: flex;
    flex-direction: column;
    gap: 0.6rem;
}

.ref-item {
    display: flex;
    flex-direction: column;
    gap: 0.25rem;
}

.ref-label {
    font-size: 0.75rem;
    color: var(--fd-text-muted);
    text-transform: uppercase;
    letter-spacing: 0.5px;
}

.ref-value {
    font-size: 0.85rem;
    color: var(--fd-text);
}

/* ===== Modal ===== */
.fd-modal {
    position: fixed;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    background: rgba(0, 0, 0, 0.6);
    display: none;
    align-items: center;
    justify-content: center;
    z-index: 9999;
    padding: 1rem;
    backdrop-filter: blur(4px);
}

.fd-modal.active {
    display: flex;
}

.fd-modal .modal-content {
    background: var(--fd-bg);
    border-radius: var(--fd-radius-lg);
    max-width: 600px;
    width: 100%;
    max-height: 90vh;
    overflow: hidden;
    display: flex;
    flex-direction: column;
    animation: modalSlide 0.3s ease;
    border: 1px solid var(--fd-border);
}

.fd-modal .modal-content.large {
    max-width: 800px;
}

@keyframes modalSlide {
    from {
        opacity: 0;
        transform: translateY(-20px) scale(0.95);
    }

    to {
        opacity: 1;
        transform: translateY(0) scale(1);
    }
}

.fd-modal .modal-header {
    display: flex;
    justify-content: space-between;
    align-items: center;
    padding: 1.25rem 1.5rem;
    border-bottom: 1px solid var(--fd-border);
}

.fd-modal .modal-header h3 {
    display: flex;
    align-items: center;
    gap: 0.5rem;
    margin: 0;
    font-size: 1.1rem;
    color: var(--fd-text);
}

.fd-modal .modal-header h3 i {
    color: var(--fd-primary);
}

.modal-close {
    width: 36px;
    height: 36px;
    display: flex;
    align-items: center;
    justify-content: center;
    border: none;
    background: var(--fd-surface);
    color: var(--fd-text);
    border-radius: 50%;
    font-size: 1.5rem;
    cursor: pointer;
    transition: var(--fd-transition);
}

.modal-close:hover {
    background: rgba(239, 68, 68, 0.1);
    color: #ef4444;
}

.fd-modal .modal-body {
    padding: 1.5rem;
    overflow-y: auto;
}

/* Export Options */
.export-options {
    display: grid;
    grid-template-columns: repeat(2, 1fr);
    gap: 1rem;
    margin-bottom: 1.5rem;
}

.export-btn {
    display: flex;
    flex-direction: column;
    align-items: center;
    gap: 0.5rem;
    padding: 1.25rem;
    border: 2px solid var(--fd-border);
    border-radius: var(--fd-radius);
    background: var(--fd-surface);
    cursor: pointer;
    transition: var(--fd-transition);
}

.export-btn:hover {
    border-color: var(--fd-primary);
    background: rgba(var(--fd-primary-rgb), 0.05);
    transform: translateY(-2px);
}

.export-btn i {
    font-size: 1.75rem;
    color: var(--fd-primary);
}

.export-btn span {
    font-size: 0.95rem;
    font-weight: 600;
    color: var(--fd-text);
}

.export-btn small {
    font-size: 0.75rem;
    color: var(--fd-text-muted);
}

.export-preview {
    background: var(--fd-surface);
    border: 1px solid var(--fd-border);
    border-radius: var(--fd-radius);
    padding: 1rem;
    font-family: 'Monaco', 'Menlo', 'Consolas', monospace;
    font-size: 0.85rem;
    max-height: 300px;
    overflow-y: auto;
    white-space: pre-wrap;
    color: var(--fd-text);
}

/* ===== Toast ===== */
.fd-toast {
    position: fixed;
    bottom: 2rem;
    right: 2rem;
    display: flex;
    align-items: center;
    gap: 0.75rem;
    padding: 1rem 1.5rem;
    background: var(--fd-success);
    color: white;
    border-radius: var(--fd-radius);
    box-shadow: var(--fd-shadow-lg);
    transform: translateY(100px);
    opacity: 0;
    transition: all 0.3s ease;
    z-index: 10000;
}

.fd-toast.show {
    transform: translateY(0);
    opacity: 1;
}

.fd-toast.error {
    background: var(--fd-danger);
}

.fd-toast.warning {
    background: var(--fd-warning);
}

/* ===== Responsive Design ===== */
@media (max-width: 1400px) {
    .fd-layout {
        grid-template-columns: 260px 1fr;
    }

    .fd-quick-ref {
        display: none;
    }
}

@media (max-width: 1024px) {
    .fd-layout {
        grid-template-columns: 1fr;
    }

    .fd-sidebar {
        position: static;
        order: 1;
    }

    .fd-main {
        order: 2;
    }
}

@media (max-width: 768px) {
    .formula-derivation-container {
        padding: 0 0.5rem;
    }

    .fd-header {
        flex-direction: column;
        text-align: center;
        padding: 1.25rem;
    }

    .fd-header .header-content {
        flex-direction: column;
    }

    .fd-header .header-icon {
        width: 50px;
        height: 50px;
        font-size: 1.25rem;
    }

    .fd-header .header-text h2 {
        font-size: 1.2rem;
    }

    .header-badges {
        justify-content: center;
    }

    .fd-mode-toggle {
        flex-direction: column;
    }

    .mode-btn {
        width: 100%;
        justify-content: center;
    }

    .fd-student-mode {
        flex-direction: column;
        align-items: stretch;
    }

    .mode-switch {
        justify-content: center;
    }

    .mode-options {
        justify-content: center;
    }

    .export-options {
        grid-template-columns: 1fr;
    }

    .panel-header {
        flex-direction: column;
        align-items: flex-start;
    }

    .derivation-step {
        flex-direction: column;
        align-items: flex-start;
    }

    .step-number {
        width: 32px;
        height: 32px;
        font-size: 0.85rem;
    }

    .step-equation {
        font-size: 1.1rem;
    }

    .formula-equation {
        font-size: 1.25rem;
    }

    .qed-box {
        flex-direction: column;
        text-align: center;
    }

    .explorer-grid {
        grid-template-columns: 1fr;
    }

    .fd-toast {
        left: 1rem;
        right: 1rem;
        bottom: 1rem;
    }
}

@media (max-width: 480px) {
    .mode-btn span {
        display: none;
    }

    .mode-btn i {
        font-size: 1.1rem;
    }

    .mode-btn {
        padding: 0.7rem 1rem;
    }

    .student-btn {
        padding: 0.4rem 0.75rem;
        font-size: 0.75rem;
    }

    .toggle-option {
        font-size: 0.75rem;
    }

    .fd-sidebar {
        padding: 1rem;
    }

    .fd-main {
        padding: 1rem;
    }
}

/* ===== Scrollbar Styling ===== */
.formula-derivation-container ::-webkit-scrollbar {
    width: 6px;
    height: 6px;
}

.formula-derivation-container ::-webkit-scrollbar-track {
    background: var(--fd-surface);
    border-radius: 3px;
}

.formula-derivation-container ::-webkit-scrollbar-thumb {
    background: rgba(var(--fd-primary-rgb), 0.3);
    border-radius: 3px;
}

.formula-derivation-container ::-webkit-scrollbar-thumb:hover {
    background: var(--fd-primary);
}

/* ===== Print Styles ===== */
@media print {

    .fd-header,
    .fd-sidebar,
    .fd-quick-ref,
    .fd-mode-toggle,
    .fd-student-mode,
    .header-actions {
        display: none !important;
    }

    .fd-layout {
        display: block !important;
    }

    .fd-main {
        border: none !important;
        padding: 0 !important;
    }

    .derivation-step {
        break-inside: avoid;
        page-break-inside: avoid;
    }
}