/* 1. Variables and Reset */
:root {
    --primary-color: #6366f1; /* Quizlet-like Indigo */
    --primary-hover: #4f46e5;
    --background-color: #f9fafb; /* Lighter Gray */
    --surface-color: #ffffff;
    --text-primary: #111827; /* Darker Gray */
    --text-secondary: #6b7281; /* Medium Gray */
    --border-color: #e5e7eb; /* Lighter Gray */
    --success-color: #10b981;
    --error-color: #ef4444;
}

[data-theme="dark"] {
    --primary-color: #818cf8; /* Lighter Indigo for dark mode */
    --primary-hover: #a5b4fc;
    --background-color: #0f172a; /* Dark Slate 900 */
    --surface-color: #1e293b; /* Dark Slate 800 */
    --text-primary: #f8fafc;     /* Off-white */
    --text-secondary: #94a3b8;   /* Slate 400 */
    --border-color: #334155;     /* Slate 700 */
}

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

body {
    font-family: 'Inter', -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, sans-serif;
    line-height: 1.7;
    color: var(--text-secondary);
    background-color: var(--background-color);
    min-height: 100vh;
}

h1, h2, h3, h4 {
    color: var(--text-primary);
    font-weight: 600;
    line-height: 1.3;
}

.container {
    max-width: 1200px;
    margin: 0 auto;
    padding: 0 20px;
}

/* 2. Header & Navigation */
.header {
    background: rgba(255, 255, 255, 0.8);
    backdrop-filter: blur(10px);
    border-bottom: 1px solid var(--border-color);
    position: sticky;
    top: 0;
    z-index: 100;
}

.nav {
    display: flex;
    justify-content: space-between;
    align-items: center;
    padding: 1.25rem 2rem;
    max-width: 1200px;
    margin: 0 auto;
}

.nav-brand a {
    display: flex;
    align-items: center;
    gap: 0.75rem;
    text-decoration: none;
    color: inherit;
}

.nav-brand h1 {
    color: var(--text-primary);
    font-size: 1.5rem;
    font-weight: 600;
    letter-spacing: -0.025em;
    margin: 0;
}

.nav-menu {
    display: flex;
    list-style: none;
    gap: 2.5rem;
}

.nav-menu a {
    text-decoration: none;
    color: var(--text-secondary);
    font-weight: 500;
    font-size: 1rem;
    transition: color 0.2s ease;
    padding: 0.5rem 0;
}

.nav-menu a:hover {
    color: var(--primary-color);
}

.theme-toggle {
    background: none;
    border: none;
    cursor: pointer;
    color: var(--text-secondary);
    transition: color 0.2s ease;
    padding: 0.5rem;
    display: flex;
    align-items: center;
    justify-content: center;
}
.theme-toggle:hover {
    color: var(--primary-color);
}
.theme-toggle .moon-icon { display: none; }
.theme-toggle .sun-icon { display: block; }
[data-theme="dark"] .theme-toggle .moon-icon { display: block; }
[data-theme="dark"] .theme-toggle .sun-icon { display: none; }
}

/* 3. Hero Section */
.hero {
    background: var(--surface-color);
    color: var(--text-primary);
    padding: 6rem 0;
    text-align: center;
    border-bottom: 1px solid var(--border-color);
    overflow: hidden; /* Contain animations */
}

.hero h1 {
    font-size: 3.5rem;
    font-weight: 700;
    margin-bottom: 1.5rem;
    line-height: 1.2;
    letter-spacing: -0.03em;
    opacity: 0;
    max-width: 750px; /* Constrain width for centering */
    margin: 0 auto 1.5rem; /* Center the block and handle bottom margin */
    animation: fadeInUp 0.8s 0.2s forwards ease-out;
}

.hero p {
    font-size: 1.25rem;
    margin-bottom: 2.5rem;
    color: var(--text-secondary);
    max-width: 650px;
    margin-left: auto;
    margin-right: auto;
    font-weight: 400;
    opacity: 0;
    animation: fadeInUp 0.8s 0.4s forwards ease-out;
}

.hero-cta {
    display: flex;
    gap: 1rem;
    justify-content: center;
    flex-wrap: wrap;
    opacity: 0;
    animation: fadeInUp 0.8s 0.6s forwards ease-out;
}

/* 4. Buttons */
.btn-primary, .btn-secondary {
    padding: 1rem 2.25rem;
    border-radius: 8px;
    text-decoration: none;
    font-weight: 500;
    font-size: 1rem;
    transition: all 0.2s ease;
    display: inline-block;
    border: 1px solid transparent;
    cursor: pointer;
}

.btn-primary {
    background: var(--primary-color);
    color: white;
}

.btn-primary:hover {
    background: var(--primary-hover);
    transform: translateY(-2px);
    box-shadow: 0 4px 12px rgba(0, 0, 0, 0.1);
}

.btn-secondary {
    background: var(--surface-color);
    color: var(--text-primary);
    border: 1px solid var(--border-color);
}

.btn-secondary:hover {
    background-color: var(--background-color);
    border-color: #ced4da;
    transform: translateY(-2px);
}

/* 5. Sections & Cards */
.tools-section, .features-section, .info-section, .faq-section, .tips-section, .related-tools {
    padding: 5rem 0;
}

.tools-section, .related-tools {
    background-color: var(--background-color);
}

.features-section, .info-section {
    background-color: var(--surface-color);
    border-top: 1px solid var(--border-color);
    border-bottom: 1px solid var(--border-color);
}

.tools-section h2, .features-section h2, .info-section h2, .faq-section h2, .tips-section h2, .related-tools h2 {
    text-align: center;
    font-size: 2.5rem;
    margin-bottom: 3.5rem;
    color: var(--text-primary);
    font-weight: 700;
    letter-spacing: -0.025em;
}

.tools-grid, .features-grid, .faq-grid, .tips-grid {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(320px, 1fr));
    gap: 2rem;
}

.tool-card, .feature, .faq-item, .tip-card {
    background: var(--surface-color);
    border-radius: 12px;
    padding: 2.5rem;
    border: 1px solid var(--border-color);
    transition: all 0.3s ease-in-out;
    display: flex;
    flex-direction: column;
}

.tool-card:hover, .feature:hover, .faq-item:hover, .tip-card:hover {
    transform: translateY(-5px);
    box-shadow: 0 10px 20px rgba(0, 0, 0, 0.07);
    border-color: var(--primary-color);
}

.tool-icon, .feature-icon {
    font-size: 2.5rem;
    margin-bottom: 1.5rem;
    color: var(--primary-color);
}

.tool-card h3, .feature h3, .faq-item h3, .tip-card h3 {
    font-size: 1.25rem;
    margin-bottom: 1rem;
    color: var(--text-primary);
    font-weight: 600;
}

.tool-card p, .feature p, .faq-item p, .tip-card p {
    color: var(--text-secondary);
    margin-bottom: 1.5rem;
    flex-grow: 1;
}

.tool-link {
    display: inline-block;
    background: var(--primary-color);
    color: white;
    padding: 0.75rem 1.5rem;
    border-radius: 6px;
    text-decoration: none;
    font-weight: 500;
    font-size: 0.9rem;
    transition: all 0.2s ease;
    margin-top: auto;
    align-self: flex-start;
}

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

/* 6. Footer */
.footer {
    background: var(--text-primary);
    color: #a0aec0;
    padding: 4rem 0 2rem;
}

.footer-content {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(220px, 1fr));
    gap: 2.5rem;
    margin-bottom: 3rem;
}

.footer-section h3, .footer-section h4 {
    color: var(--surface-color);
    margin-bottom: 1.25rem;
    font-weight: 600;
}

.footer-section ul {
    list-style: none;
}

.footer-section ul li {
    margin-bottom: 0.75rem;
}

.footer-section a {
    color: #a0aec0;
    text-decoration: none;
    transition: color 0.2s ease;
}

.footer-section a:hover {
    color: var(--surface-color);
}

.footer-bottom {
    border-top: 1px solid #475569;
    padding-top: 2rem;
    text-align: center;
    color: #718096;
}

/* 7. Tool Page Specific Styles */

/* Tool Hero */
.tool-hero {
    background: var(--primary-color);
    color: white;
    padding: 4rem 0;
    text-align: center;
}

.tool-hero h1 {
    font-size: 3rem;
    font-weight: 700;
    margin-bottom: 1rem;
    letter-spacing: -0.025em;
    color: white;
}

.tool-hero p {
    font-size: 1.2rem;
    opacity: 0.9;
    font-weight: 300;
    max-width: 700px;
    margin: 0 auto;
}

/* Calculator & Form Styles */
.calculator-section {
    padding: 4rem 0;
    background: var(--background-color);
}

.calculator-container {
    max-width: 800px;
    margin: 0 auto;
    background: var(--surface-color);
    border-radius: 12px;
    padding: 2.5rem;
    border: 1px solid var(--border-color);
    box-shadow: 0 8px 25px rgba(226, 232, 240, 0.5);
}

.form-group {
    margin-bottom: 1.5rem;
}

.form-group label {
    display: block;
    margin-bottom: 0.5rem;
    font-weight: 500;
    color: var(--text-primary);
}

.form-group input, .form-group select, .form-group textarea {
    width: 100%;
    padding: 0.75rem;
    border: 1px solid var(--border-color);
    border-radius: 8px;
    background: var(--surface-color);
    font-size: 1rem;
    transition: border-color 0.2s ease, box-shadow 0.2s ease;
}

.form-group input:focus, .form-group select:focus, .form-group textarea:focus {
    outline: none;
    border-color: var(--primary-color);
    box-shadow: 0 0 0 3px rgba(59, 130, 246, 0.2);
}

.calculator-actions {
    margin-top: 2rem;
    display: flex;
    gap: 1rem;
    justify-content: center;
    flex-wrap: wrap;
}

.results-container {
    background: var(--background-color);
    border-radius: 8px;
    padding: 1.5rem;
    margin-top: 2rem;
    border: 1px solid var(--border-color);
}

.results-container h3 {
    margin-bottom: 1rem;
}

.result-item {
    display: flex;
    justify-content: space-between;
    margin-bottom: 0.75rem;
    font-size: 1.1rem;
    padding: 0.5rem 0;
    border-bottom: 1px solid var(--border-color);
}
.result-item:last-child {
    border-bottom: none;
}

.result-value {
    font-weight: 600;
    color: var(--primary-color);
}

/* GPA Calculator */
.course-row {
    display: grid;
    grid-template-columns: 2fr 1fr 1.5fr auto;
    gap: 1rem;
    margin-bottom: 1rem;
    align-items: center;
}

.remove-course {
    background: var(--error-color);
    color: white;
    border: none;
    border-radius: 50%;
    width: 32px;
    height: 32px;
    cursor: pointer;
    font-size: 1.2rem;
    transition: background 0.2s ease;
    display: flex;
    align-items: center;
    justify-content: center;
    padding: 0;
    min-width: 32px;
}

.remove-course:hover {
    background: #c53030;
}

.grade-table {
    margin-top: 2rem;
    border: 1px solid var(--border-color);
    border-radius: 8px;
    overflow: hidden;
}

.grade-row {
    display: grid;
    grid-template-columns: 2fr 1fr 1fr;
    padding: 0.75rem 1rem;
    border-bottom: 1px solid var(--border-color);
}
.grade-row:last-child {
    border-bottom: none;
}

.grade-row:first-child {
    font-weight: 600;
    background: var(--background-color);
}

/* Pomodoro Timer */
.timer-container {
    max-width: 500px;
    margin: 0 auto;
    text-align: center;
}

.timer-circle {
    position: relative;
    display: inline-block;
    margin-bottom: 2rem;
}

.progress-ring {
    transform: rotate(-90deg);
}

.progress-ring-circle {
    stroke: var(--border-color);
}

.progress-ring-progress {
    stroke: var(--primary-color);
    transition: stroke-dashoffset 0.35s;
}

.timer-content {
    position: absolute;
    top: 50%;
    left: 50%;
    transform: translate(-50%, -50%);
}

.timer-mode {
    font-size: 1.2rem;
    color: var(--text-secondary);
    margin-bottom: 0.5rem;
}

.timer-time {
    font-size: 4rem;
    font-weight: 700;
    color: var(--text-primary);
    margin-bottom: 0.5rem;
}

.timer-session {
    font-size: 1rem;
    color: var(--text-secondary);
}

.timer-controls {
    margin-bottom: 3rem;
    display: flex;
    gap: 1rem;
    justify-content: center;
}

.timer-settings, .timer-stats {
    background: var(--background-color);
    padding: 2rem;
    border-radius: 12px;
    margin-bottom: 2rem;
    text-align: left;
}

/* New Pomodoro Settings Styles */
.new-settings-grid {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(150px, 1fr));
    gap: 1.5rem;
    margin-bottom: 2rem;
}
.new-setting-item label {
    font-size: 0.9rem;
    color: var(--text-secondary);
    margin-bottom: 0.5rem;
    display: block;
}
.input-suffix {
    display: flex;
    align-items: center;
    border: 1px solid var(--border-color);
    border-radius: 8px;
    background: var(--surface-color);
}
.input-suffix input {
    border: none;
    background: transparent;
    padding: 0.75rem;
    width: 100%;
    outline: none;
}
.input-suffix span {
    padding-right: 0.75rem;
    font-size: 0.9rem;
    color: var(--text-secondary);
}
.new-settings-toggles {
    display: flex;
    flex-direction: column;
    gap: 1rem;
}
.toggle-item {
    display: flex;
    justify-content: space-between;
    align-items: center;
    padding: 1rem;
    background: var(--background-color);
    border-radius: 8px;
}
.toggle-switch { position: relative; display: inline-block; width: 44px; height: 24px; }
.toggle-switch input { opacity: 0; width: 0; height: 0; }
.slider { position: absolute; cursor: pointer; top: 0; left: 0; right: 0; bottom: 0; background-color: #ccc; transition: .4s; border-radius: 24px; }
.slider:before { position: absolute; content: ""; height: 18px; width: 18px; left: 3px; bottom: 3px; background-color: white; transition: .4s; border-radius: 50%; }
.toggle-switch input:checked + .slider { background-color: var(--primary-color); }
.toggle-switch input:focus + .slider { box-shadow: 0 0 1px var(--primary-color); }
.toggle-switch input:checked + .slider:before {
    transform: translateX(20px);
}

/* Citation Generator */
.citation-container {
    max-width: 900px;
    margin: 0 auto;
}

.citation-settings, .citation-form, .citation-result, .bibliography-section {
    background: var(--surface-color);
    border-radius: 12px;
    padding: 2rem;
    margin-bottom: 2rem;
    border: 1px solid var(--border-color);
}

.citation-output {
    background: var(--background-color);
    border: 1px solid var(--border-color);
    border-radius: 8px;
    padding: 1.5rem;
    position: relative;
}

.citation-text {
    font-family: 'Times New Roman', serif;
    line-height: 2;
    margin-bottom: 1rem;
    padding: 1rem;
    background: var(--surface-color);
    border-radius: 6px;
}

.bibliography-list {
    min-height: 100px;
    background: var(--surface-color);
    border-radius: 8px;
    padding: 1rem;
    margin-bottom: 1rem;
    border: 1px solid var(--border-color);
}

.bibliography-item {
    padding: 0.5rem;
    border-bottom: 1px solid var(--border-color);
    font-family: 'Times New Roman', serif;
    line-height: 2;
}
.bibliography-item:last-child {
    border-bottom: none;
}

.bibliography-actions {
    display: flex;
    gap: 1rem;
    margin-top: 1rem;
}

.guide-tabs {
    display: flex;
    gap: 1rem;
    margin-bottom: 2rem;
    justify-content: center;
    flex-wrap: wrap;
    border-bottom: 1px solid var(--border-color);
}

.tab-button {
    padding: 0.75rem 1.5rem;
    border: none;
    border-bottom: 2px solid transparent;
    background: transparent;
    border-radius: 0;
    cursor: pointer;
    font-weight: 500;
    transition: all 0.2s ease;
    color: var(--text-secondary);
}

.tab-button.active {
    color: var(--primary-color);
    border-bottom-color: var(--primary-color);
}

.tab-panel {
    display: none;
}

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

/* Study Planner */
.planner-section {
    padding: 4rem 0;
    background: var(--background-color);
}
.planner-container {
    display: grid;
    grid-template-columns: 350px 1fr;
    gap: 2rem;
    max-width: 1400px;
    margin: 0 auto;
}
.planner-sidebar {
    background: var(--surface-color);
    border-radius: 12px;
    padding: 1.5rem;
    height: fit-content;
    position: sticky;
    top: 120px;
    border: 1px solid var(--border-color);
}
.sidebar-section {
    margin-bottom: 2rem;
}
.sidebar-section:last-child {
    margin-bottom: 0;
}
.stats-grid {
    display: grid;
    grid-template-columns: 1fr;
    gap: 1rem;
}
.stat-item {
    text-align: center;
    padding: 1rem;
    background: var(--background-color);
    border-radius: 8px;
    border: 1px solid var(--border-color);
}
.stat-number {
    display: block;
    font-size: 1.5rem;
    font-weight: 700;
    color: var(--primary-color);
}
.stat-label {
    font-size: 0.8rem;
    color: var(--text-secondary);
}
.planner-main {
    background: var(--surface-color);
    border-radius: 12px;
    padding: 1.5rem;
    border: 1px solid var(--border-color);
}
.planner-header {
    display: flex;
    justify-content: space-between;
    align-items: center;
    margin-bottom: 1.5rem;
}
.view-controls {
    display: flex;
    gap: 0.5rem;
}
.view-btn {
    padding: 0.5rem 1rem;
    border: 1px solid var(--border-color);
    background: var(--surface-color);
    border-radius: 6px;
    cursor: pointer;
    font-size: 0.9rem;
}
.view-btn.active {
    background: var(--primary-color);
    color: white;
    border-color: var(--primary-color);
}
.calendar-header {
    display: flex;
    justify-content: space-between;
    align-items: center;
    margin-bottom: 1rem;
}
.nav-btn {
    background: var(--primary-color);
    color: white;
    border: none;
    border-radius: 50%;
    width: 40px;
    height: 40px;
    cursor: pointer;
    font-size: 1.2rem;
}
.calendar-grid {
    display: grid;
    grid-template-columns: 60px 1fr;
    background: var(--surface-color);
    border-radius: 8px;
    overflow: hidden;
    border: 1px solid var(--border-color);
}
.time-column {
    background: var(--background-color);
    border-right: 1px solid var(--border-color);
}
.time-slot {
    height: 60px;
    display: flex;
    align-items: center;
    justify-content: center;
    font-size: 0.8rem;
    color: var(--text-secondary);
    border-bottom: 1px solid var(--border-color);
}
.calendar-days {
    display: grid;
    grid-template-columns: repeat(7, 1fr);
}
.day-column {
    border-right: 1px solid var(--border-color);
    position: relative;
}
.day-column:last-child { border-right: none; }
.day-header {
    height: 40px;
    display: flex;
    align-items: center;
    justify-content: center;
    font-weight: 600;
    background: var(--background-color);
    border-bottom: 1px solid var(--border-color);
    font-size: 0.9rem;
}
.day-slots { position: relative; }
.hour-slot {
    height: 60px;
    border-bottom: 1px solid #f1f5f9;
    position: relative;
}
.study-session {
    position: absolute;
    left: 2px;
    right: 2px;
    background: var(--primary-color);
    color: white;
    border-radius: 4px;
    padding: 4px 6px;
    font-size: 0.7rem;
    cursor: pointer;
    overflow: hidden;
    z-index: 1;
}
.study-session.high { background: var(--error-color); }
.study-session.medium { background: #f59e0b; }
.study-session.low { background: var(--success-color); }
.study-session.completed { opacity: 0.6; text-decoration: line-through; }
.list-view {
    background: var(--surface-color);
    border-radius: 8px;
    padding: 1rem;
}
.list-filters {
    display: flex;
    gap: 1rem;
    margin-bottom: 1rem;
}
.session-item {
    display: flex;
    justify-content: space-between;
    align-items: center;
    padding: 1rem;
    border: 1px solid var(--border-color);
    border-radius: 8px;
    margin-bottom: 0.5rem;
    background: var(--surface-color);
}
.session-info h4 { margin: 0 0 0.25rem 0; color: var(--text-primary); }
.session-info p { margin: 0; color: var(--text-secondary); font-size: 0.9rem; }
.session-actions { display: flex; gap: 0.5rem; }
.action-btn { padding: 0.25rem 0.5rem; border: none; border-radius: 4px; cursor: pointer; font-size: 0.8rem; }
.complete-btn { background: var(--success-color); color: white; }
.delete-btn { background: var(--error-color); color: white; }

/* Citation Generator Workspace Layout */
.citation-workspace {
    display: grid;
    grid-template-columns: 1fr 1fr;
    gap: 2.5rem;
    align-items: flex-start;
}

.citation-output-container {
    position: sticky;
    top: 120px; /* Adjust based on header height */
}

.citation-actions {
    display: flex;
    gap: 1rem;
    margin-top: 1rem;
}
.citation-actions .btn-primary, .citation-actions .btn-secondary {
    flex-grow: 1;
}

/* Percentage Calculator */
.form-row {
    display: flex;
    gap: 1rem;
    align-items: end;
    flex-wrap: wrap;
}
.result-display {
    margin-top: 1.5rem;
}
.result-card {
    background: var(--background-color);
    padding: 1.5rem;
    border-radius: 8px;
    border: 1px solid var(--border-color);
}
.result-card h4 {
    color: var(--primary-color);
    margin-bottom: 0.5rem;
    font-size: 1.25rem;
}
.error {
    color: var(--error-color);
    font-weight: 500;
}

/* Unit Converter */
.converter-row {
    display: flex;
    align-items: center;
    gap: 1rem;
    margin: 2rem 0;
    flex-wrap: wrap;
}
.converter-input {
    display: flex;
    flex-direction: column;
    gap: 0.5rem;
    flex: 1;
    min-width: 200px;
}
.converter-equals {
    font-size: 1.5rem;
    font-weight: 600;
    color: var(--text-secondary);
    margin: 0 1rem;
}

/* Random Group Generator */
.settings-row {
    display: flex;
    gap: 1rem;
    margin-bottom: 1rem;
}
.settings-row .form-group { flex: 1; }
.groups-container { margin-top: 2rem; }
.group-card {
    background: var(--background-color);
    border: 1px solid var(--border-color);
    border-radius: 8px;
    padding: 1.5rem;
    margin-bottom: 1rem;
}
.group-card h4 { margin-bottom: 1rem; }
.members-list { list-style: none; }
.members-list li { padding: 0.5rem 0; border-bottom: 1px solid var(--border-color); }
.members-list li:last-child { border-bottom: none; }
.result-actions { display: flex; gap: 1rem; justify-content: center; margin-top: 1.5rem; }

/* About/Contact/Privacy Pages */
.about-content h2, .privacy-content h2 { 
    margin-bottom: 1rem; 
    font-size: 2rem; 
}
.about-content p, .privacy-content p, .privacy-content ul { 
    margin-bottom: 1.5rem; 
    line-height: 1.8; 
    color: var(--text-secondary);
}
.privacy-content ul { 
    padding-left: 2rem; 
}

/* Ad Container */
.ad-container {
    max-width: 1200px;
    margin: 2rem auto;
    padding: 0 20px;
    text-align: center;
}

.adsbygoogle {
    display: block;
    margin: 1rem 0;
}

/* Responsive Design */
@media (max-width: 768px) {
    .nav {
        flex-direction: column;
        gap: 1rem;
        padding: 1rem;
    }
    
    .nav-menu {
        gap: 1.5rem;
    }
    
    .hero {
        padding: 4rem 0;
    }
    
    .hero h1 {
        font-size: 2.5rem;
    }
    
    .hero-cta {
        flex-direction: column;
        align-items: center;
    }
    
    .tools-grid, .features-grid, .faq-grid, .tips-grid {
        grid-template-columns: 1fr;
    }
    
    .course-row {
        grid-template-columns: 1fr;
        gap: 0.5rem;
    }
    
    .calculator-actions {
        flex-direction: column;
    }

    .planner-container {
        grid-template-columns: 1fr;
    }
    
    .planner-sidebar {
        position: static;
    }

    .citation-workspace {
        grid-template-columns: 1fr;
    }

    .citation-output-container {
        position: static;
        top: auto;
    }

    .converter-row {
        flex-direction: column;
    }
    .converter-equals {
        transform: rotate(90deg);
        margin: 0.5rem 0;
    }

    .settings-row {
        flex-direction: column;
    }
    .result-actions {
        flex-direction: column;
    }

    .calendar-days {
        grid-template-columns: repeat(3, 1fr);
    }
    .list-filters { flex-direction: column; }
    .session-item { flex-direction: column; align-items: flex-start; gap: 1rem; }
}

/* 8. Animations */
@keyframes fadeInUp {
    from {
        opacity: 0;
        transform: translateY(20px);
    }
    to {
        opacity: 1;
        transform: translateY(0);
    }
}


/* Back to Top Button */
#backToTopBtn {
    position: fixed;
    bottom: 30px;
    right: 30px;
    z-index: 101; /* Higher than header */
    border: none;
    outline: none;
    background-color: var(--primary-color);
    color: white;
    cursor: pointer;
    padding: 0;
    border-radius: 50%;
    width: 50px;
    height: 50px;
    font-size: 24px;
    box-shadow: 0 4px 12px rgba(0, 0, 0, 0.15);
    transition: opacity 0.3s, visibility 0.3s, transform 0.3s;
    opacity: 0;
    visibility: hidden;
    transform: translateY(20px);
}

#backToTopBtn.show {
    opacity: 1;
    visibility: visible;
    transform: translateY(0);
}

#backToTopBtn:hover {
    background-color: var(--primary-hover);
}

/* Error Page */
.error-page-hero {
    padding: 6rem 0;
    text-align: center;
}
.error-page-hero p {
    max-width: 700px;
    margin: 1.5rem auto 2.5rem;
}
.error-page-hero .btn-primary {
    background: var(--surface-color);
    color: var(--primary-color);
}
.error-page-hero .btn-primary:hover {
    background: var(--background-color);
}
.error-page-subheading {
    text-align: center;
    margin-bottom: 2rem;
}
.grade-message {
    margin-top: 1rem;
    font-weight: 500;
    text-align: center;
    color: var(--text-secondary);
}

/* Vocabulary Builder - Multiple Choice Options */
.options-grid {
    display: grid;
    grid-template-columns: 1fr 1fr;
    gap: 1rem;
    margin-top: 2rem;
}

.option-btn {
    padding: 1rem;
    border: 1px solid var(--border-color);
    background: var(--surface-color);
    color: var(--text-primary);
    border-radius: 8px;
    font-size: 1rem;
    font-weight: 500;
    cursor: pointer;
    transition: all 0.2s ease;
    text-align: left;
    width: 100%;
}

.option-btn:hover:not(:disabled) {
    border-color: var(--primary-color);
    transform: translateY(-2px);
    box-shadow: 0 4px 12px rgba(0, 0, 0, 0.05);
}

.option-btn.correct { background-color: var(--success-color); color: white; border-color: var(--success-color); }
.option-btn.incorrect { background-color: var(--error-color); color: white; border-color: var(--error-color); }
.option-btn:disabled { cursor: not-allowed; }

@media (max-width: 768px) {
    .options-grid { grid-template-columns: 1fr; }
}