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

:root {
    --bg-primary: #0f172a;
    --bg-secondary: #1e293b;
    --bg-tertiary: #334155;
    --text-primary: #f1f5f9;
    --text-secondary: #cbd5e1;
    --accent: #3b82f6;
    --accent-hover: #2563eb;
    --success: #10b981;
    --error: #ef4444;
    --warning: #f59e0b;
    --border: #475569;
    
    /* Typography scale for consistency */
    --font-size-xs: 0.75rem;
    --font-size-sm: 0.875rem;
    --font-size-base: 1rem;
    --font-size-lg: 1.125rem;
    --font-size-xl: 1.25rem;
    --font-size-2xl: 1.5rem;
    --font-size-3xl: 1.875rem;
    --font-size-4xl: 2.25rem;
    
    /* Line heights for better readability */
    --line-height-tight: 1.25;
    --line-height-normal: 1.5;
    --line-height-relaxed: 1.75;
}

body {
    font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, Oxygen, Ubuntu, Cantarell, 'Open Sans', 'Helvetica Neue', sans-serif;
    background: var(--bg-primary);
    color: var(--text-primary);
    line-height: 1.6;
    transition: all 0.3s ease;
    min-height: 100vh;
    display: flex;
    flex-direction: column;
}

body.theme-ocean {
    --bg-primary: #0c4a6e;
    --bg-secondary: #075985;
    --bg-tertiary: #0369a1;
    --accent: #06b6d4;
    --accent-hover: #0891b2;
}

body.theme-forest {
    --bg-primary: #14532d;
    --bg-secondary: #166534;
    --bg-tertiary: #15803d;
    --accent: #22c55e;
    --accent-hover: #16a34a;
}

body.theme-sunset {
    --bg-primary: #7c2d12;
    --bg-secondary: #9a3412;
    --bg-tertiary: #c2410c;
    --accent: #f97316;
    --accent-hover: #ea580c;
}

body.theme-purple {
    --bg-primary: #4c1d95;
    --bg-secondary: #5b21b6;
    --bg-tertiary: #6d28d9;
    --accent: #a855f7;
    --accent-hover: #9333ea;
}

body.theme-light {
    --bg-primary: #f8fafc;
    --bg-secondary: #f1f5f9;
    --bg-tertiary: #e2e8f0;
    --text-primary: #0f172a;
    --text-secondary: #475569;
    --accent: #3b82f6;
    --accent-hover: #2563eb;
    --border: #cbd5e1;
}

.container {
    max-width: 1200px;
    margin: 0 auto;
    padding: 20px;
    flex: 1;
    width: 100%;
}

header {
    background: var(--bg-secondary);
    padding: 20px 0;
    border-bottom: 2px solid var(--border);
    margin-bottom: 30px;
    box-shadow: 0 4px 6px rgba(0, 0, 0, 0.1);
}

.header-content {
    display: flex;
    justify-content: space-between;
    align-items: center;
    flex-wrap: wrap;
    gap: 20px;
    min-height: 80px;
}

h1 {
    font-size: 2.5rem;
    background: linear-gradient(135deg, var(--accent), var(--accent-hover));
    -webkit-background-clip: text;
    -webkit-text-fill-color: transparent;
    background-clip: text;
    font-weight: 800;
}

.tagline {
    color: var(--text-secondary);
    font-size: 0.9rem;
    margin-top: 5px;
}

.tagline a {
    color: var(--accent);
    text-decoration: none;
    font-weight: 600;
    transition: all 0.2s ease;
    padding: 2px 8px;
    border-radius: 4px;
}

.tagline a:hover {
    background: rgba(59, 130, 246, 0.1);
    text-decoration: underline;
}

.theme-selector {
    display: flex;
    gap: 10px;
    align-items: center;
}

.theme-btn {
    width: 35px;
    height: 35px;
    border-radius: 50%;
    border: 2px solid var(--border);
    cursor: pointer;
    transition: all 0.2s ease;
    position: relative;
}

.theme-btn:hover {
    transform: scale(1.15);
}

.theme-btn.active {
    border-color: var(--accent);
    box-shadow: 0 0 0 3px var(--bg-primary), 0 0 0 5px var(--accent);
}

.theme-midnight { background: #0f172a; }
.theme-ocean { background: #0c4a6e; }
.theme-forest { background: #14532d; }
.theme-sunset { background: #7c2d12; }
.theme-purple { background: #4c1d95; }
.theme-light { background: #f8fafc; border-color: #94a3b8; }

.controls {
    background: var(--bg-secondary);
    padding: 25px;
    border-radius: 16px;
    margin-bottom: 30px;
    border: 1px solid var(--border);
    box-shadow: 0 4px 6px rgba(0, 0, 0, 0.1);
    display: grid;
    gap: 20px;
}

.control-group {
    display: flex;
    flex-direction: column;
    gap: 10px;
}

label {
    display: block;
    margin-bottom: 10px;
    font-weight: 700;
    color: var(--text-secondary);
    font-size: 0.95rem;
    text-transform: uppercase;
    letter-spacing: 0.5px;
}

select, button {
    width: 100%;
    padding: 14px;
    border: 2px solid var(--border);
    border-radius: 10px;
    background: var(--bg-tertiary);
    color: var(--text-primary);
    font-size: 1rem;
    cursor: pointer;
    transition: all 0.2s ease;
    font-weight: 500;
    min-height: 44px; /* Better touch target */
    display: flex;
    align-items: center;
    justify-content: center;
    gap: 8px;
}

select:hover, select:focus {
    border-color: var(--accent);
    outline: none;
    transform: translateY(-1px);
}

/* Better focus states for accessibility */
button:focus,
select:focus,
input:focus {
    outline: 2px solid var(--accent);
    outline-offset: 2px;
}

.mode-tabs {
    display: grid;
    grid-template-columns: repeat(3, 1fr);
    gap: 12px;
    margin-top: 10px;
}

.mode-tab {
    padding: 16px 20px;
    border: 2px solid var(--border);
    background: var(--bg-tertiary);
    border-radius: 10px;
    transition: all 0.2s ease;
    font-weight: 700;
    min-height: 48px; /* Better touch target */
    display: flex;
    align-items: center;
    justify-content: center;
    cursor: pointer;
}

.mode-tab:hover {
    border-color: var(--accent);
    transform: translateY(-2px);
    box-shadow: 0 4px 12px rgba(59, 130, 246, 0.2);
}

.mode-tab.active {
    background: var(--accent);
    border-color: var(--accent);
    color: white;
    box-shadow: 0 4px 12px rgba(59, 130, 246, 0.3);
}

.stats-panel {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(120px, 1fr));
    gap: 15px;
    margin-top: 20px;
    padding-top: 20px;
    border-top: 1px solid var(--border);
}

.stat-item {
    text-align: center;
    padding: 15px;
    background: var(--bg-tertiary);
    border-radius: 10px;
    border: 1px solid var(--border);
    min-height: 80px;
    display: flex;
    flex-direction: column;
    justify-content: center;
}

.stat-label {
    display: block;
    font-size: 0.8rem;
    color: var(--text-secondary);
    text-transform: uppercase;
    letter-spacing: 0.5px;
    margin-bottom: 5px;
}

.stat-value {
    display: block;
    font-size: 1.5rem;
    font-weight: 800;
    color: var(--accent);
}

.share-btn {
    grid-column: 1 / -1;
    padding: 12px 24px;
    background: linear-gradient(135deg, var(--accent), #2563eb);
    color: white;
    border: none;
    border-radius: 10px;
    font-size: 1rem;
    font-weight: 700;
    cursor: pointer;
    transition: all 0.3s ease;
    box-shadow: 0 4px 12px rgba(59, 130, 246, 0.3);
    margin-top: 5px;
}

.share-btn:hover {
    transform: translateY(-2px);
    box-shadow: 0 6px 16px rgba(59, 130, 246, 0.4);
    background: linear-gradient(135deg, #2563eb, var(--accent));
}

.share-btn:active {
    transform: translateY(0);
}

.share-btn:disabled {
    opacity: 0.5;
    cursor: not-allowed;
    transform: none;
}

.content-area {
    background: var(--bg-secondary);
    padding: 35px;
    border-radius: 16px;
    border: 1px solid var(--border);
    min-height: 500px;
    box-shadow: 0 4px 6px rgba(0, 0, 0, 0.1);
    animation: fadeIn 0.3s ease;
    max-width: 100%;
    overflow-x: auto;
}

@keyframes fadeIn {
    from {
        opacity: 0;
        transform: translateY(10px);
    }
    to {
        opacity: 1;
        transform: translateY(0);
    }
}

.lesson-content h2 {
    color: var(--accent);
    margin-bottom: 24px;
    font-size: var(--font-size-3xl);
    font-weight: 800;
    line-height: var(--line-height-tight);
}

.lesson-content h3 {
    color: var(--text-primary);
    margin-top: 32px;
    margin-bottom: 16px;
    font-size: var(--font-size-xl);
    font-weight: 700;
    line-height: var(--line-height-normal);
}

.lesson-content p {
    margin-bottom: 18px;
    color: var(--text-secondary);
    line-height: var(--line-height-relaxed);
    font-size: var(--font-size-lg);
    max-width: 70ch; /* Optimal reading width */
}

.lesson-content ul, .lesson-content ol {
    margin-left: 25px;
    margin-bottom: 15px;
    color: var(--text-secondary);
}

.lesson-content li {
    margin-bottom: 8px;
    line-height: 1.8;
}

.lesson-content code {
    background: var(--bg-tertiary);
    padding: 3px 8px;
    border-radius: 6px;
    font-family: 'SF Mono', 'Monaco', 'Inconsolata', 'Roboto Mono', 'Courier New', monospace;
    color: var(--accent);
    font-size: 0.9em;
    border: 1px solid var(--border);
}

.example {
    background: var(--bg-tertiary);
    padding: 25px;
    border-radius: 12px;
    margin: 25px 0;
    border-left: 5px solid var(--accent);
    box-shadow: 0 2px 4px rgba(0, 0, 0, 0.1);
}

.example-title {
    font-weight: 700;
    color: var(--accent);
    margin-bottom: 15px;
    font-size: 1.1rem;
}

.formula-box {
    background: var(--bg-primary);
    padding: 20px;
    border-radius: 10px;
    text-align: center;
    margin: 20px 0;
    border: 2px solid var(--accent);
    font-size: 1.3rem;
    font-family: 'SF Mono', 'Monaco', 'Inconsolata', 'Roboto Mono', 'Courier New', monospace;
    color: var(--accent);
    font-weight: 700;
    overflow-x: auto;
    word-break: break-all;
}

.problem-display {
    font-size: 2.5rem;
    text-align: center;
    margin: 50px 0;
    color: var(--text-primary);
    font-weight: 400;
    letter-spacing: 2px;
    line-height: 1.2;
    word-break: break-word;
    hyphens: auto;
}

.answer-input {
    display: flex;
    gap: 15px;
    max-width: 500px;
    margin: 40px auto;
    align-items: stretch;
}

.answer-input input {
    flex: 1;
    padding: 18px;
    border: 3px solid var(--border);
    border-radius: 12px;
    background: var(--bg-tertiary);
    color: var(--text-primary);
    font-size: 1.3rem;
    text-align: center;
    font-weight: 600;
    transition: all 0.2s ease;
    min-height: 60px;
    font-size: 16px; /* Prevents zoom on iOS */
}

.answer-input input:focus {
    outline: none;
    border-color: var(--accent);
    box-shadow: 0 0 0 3px rgba(59, 130, 246, 0.1);
}

.btn-submit {
    padding: 18px 35px;
    background: var(--accent);
    color: white;
    border: none;
    font-weight: 700;
    width: auto;
    border-radius: 12px;
    font-size: 1.1rem;
}

.btn-submit:hover {
    background: var(--accent-hover);
    transform: translateY(-3px);
    box-shadow: 0 6px 20px rgba(59, 130, 246, 0.4);
}

.feedback {
    text-align: center;
    padding: 25px;
    border-radius: 12px;
    margin: 25px 0;
    font-size: 1.2rem;
    font-weight: 700;
    animation: slideIn 0.3s ease;
}

@keyframes slideIn {
    from {
        opacity: 0;
        transform: scale(0.9);
    }
    to {
        opacity: 1;
        transform: scale(1);
    }
}

.feedback.correct {
    background: rgba(16, 185, 129, 0.15);
    color: var(--success);
    border: 3px solid var(--success);
}

.feedback.incorrect {
    background: rgba(239, 68, 68, 0.15);
    color: var(--error);
    border: 3px solid var(--error);
}

.solution {
    background: var(--bg-tertiary);
    padding: 25px;
    border-radius: 12px;
    margin-top: 25px;
    border: 2px solid var(--border);
}

.solution-title {
    font-weight: 700;
    color: var(--accent);
    margin-bottom: 15px;
    font-size: 1.2rem;
}

.btn-new {
    margin-top: 25px;
    background: var(--bg-tertiary);
    border: 3px solid var(--accent);
    color: var(--accent);
    font-weight: 700;
    font-size: 1.1rem;
}

.btn-new:hover {
    background: var(--accent);
    color: white;
    transform: translateY(-2px);
}

.walkthrough-step {
    background: var(--bg-tertiary);
    padding: 25px;
    border-radius: 12px;
    margin: 25px 0;
    border-left: 5px solid var(--accent);
    box-shadow: 0 2px 4px rgba(0, 0, 0, 0.1);
}

.step-number {
    display: inline-block;
    background: var(--accent);
    color: white;
    width: 35px;
    height: 35px;
    border-radius: 50%;
    text-align: center;
    line-height: 35px;
    margin-right: 12px;
    font-weight: 700;
    font-size: 1.1rem;
}

.progress-bar {
    width: 100%;
    height: 8px;
    background: var(--bg-tertiary);
    border-radius: 10px;
    overflow: hidden;
    margin: 20px 0;
}

.progress-fill {
    height: 100%;
    background: linear-gradient(90deg, var(--accent), var(--accent-hover));
    transition: width 0.5s ease;
    border-radius: 10px;
}

footer {
    text-align: center;
    padding: 35px 0;
    margin-top: 50px;
    color: var(--text-secondary);
    border-top: 1px solid var(--border);
    background: var(--bg-secondary);
}

/* Comprehensive responsive breakpoints */

@media (max-width: 1200px) {
    .container {
        max-width: 100%;
        padding: 15px;
    }
    
    .header-content {
        flex-direction: column;
        text-align: center;
        gap: 15px;
    }
    
    .theme-selector {
        justify-content: center;
    }
}

@media (max-width: 992px) {
    .stats-panel {
        grid-template-columns: repeat(2, 1fr);
        gap: 12px;
    }
    
    .lesson-content h2 {
        font-size: 1.6rem;
    }
    
    .content-area {
        padding: 25px;
    }
}

@media (max-width: 768px) {
    h1 {
        font-size: 1.8rem;
    }

    .mode-tabs {
        grid-template-columns: 1fr;
    }

    .problem-display {
        font-size: 1.8rem;
    }

    .stats-panel {
        grid-template-columns: 1fr;
    }

    .answer-input {
        flex-direction: column;
    }

    .btn-submit {
        width: 100%;
    }

    .content-area {
        padding: 20px;
    }
    
    .controls {
        padding: 20px;
    }
}

@media (max-width: 576px) {
    .container {
        padding: 10px;
    }
    
    h1 {
        font-size: 1.5rem;
    }
    
    .tagline {
        font-size: 0.8rem;
    }
    
    .controls {
        padding: 15px;
        margin-bottom: 20px;
    }
    
    .content-area {
        padding: 15px;
        min-height: 400px;
    }
    
    .problem-display {
        font-size: 1.5rem;
        margin: 30px 0;
    }
    
    .answer-input input {
        font-size: 1.1rem;
        padding: 15px;
    }
    
    .btn-submit {
        padding: 15px 25px;
        font-size: 1rem;
    }
    
    .theme-btn {
        width: 30px;
        height: 30px;
    }
    
    .lesson-content h2 {
        font-size: 1.4rem;
    }
    
    .lesson-content h3 {
        font-size: 1.2rem;
    }
    
    .lesson-content p {
        font-size: 1rem;
    }
}

@media (max-width: 400px) {
    .header-content {
        gap: 10px;
    }
    
    .theme-selector {
        gap: 8px;
    }
    
    .theme-btn {
        width: 28px;
        height: 28px;
    }
    
    .controls {
        padding: 12px;
    }
    
    .content-area {
        padding: 12px;
    }
    
    .problem-display {
        font-size: 1.3rem;
        letter-spacing: 1px;
    }
}

.difficulty-selector {
    display: flex;
    gap: 10px;
    margin-top: 10px;
}

.difficulty-btn {
    flex: 1;
    padding: 10px;
    border: 2px solid var(--border);
    background: var(--bg-tertiary);
    border-radius: 8px;
    font-size: 0.9rem;
    font-weight: 600;
    transition: all 0.2s ease;
}

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

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

.hint-box {
    background: var(--bg-tertiary);
    padding: 20px;
    border-radius: 10px;
    margin: 20px 0;
    border-left: 4px solid var(--warning);
}

.hint-title {
    color: var(--warning);
    font-weight: 700;
    margin-bottom: 10px;
}

.loading {
    text-align: center;
    padding: 50px;
    font-size: 1.2rem;
    color: var(--text-secondary);
}

/* Loading and skeleton states */
.loading-skeleton {
    background: linear-gradient(90deg, var(--bg-tertiary) 25%, var(--border) 50%, var(--bg-tertiary) 75%);
    background-size: 200% 100%;
    animation: loading 1.5s infinite;
}

@keyframes loading {
    0% {
        background-position: 200% 0;
    }
    100% {
        background-position: -200% 0;
    }
}

/* Improve transitions */
* {
    transition: color 0.2s ease, background-color 0.2s ease, border-color 0.2s ease;
}

/* Better hover states */
.mode-tab:hover,
.theme-btn:hover,
button:hover {
    transform: translateY(-2px);
    box-shadow: 0 4px 12px rgba(59, 130, 246, 0.2);
}

/* Floating Feedback Button */
.feedback-button {
    position: fixed;
    bottom: 20px;
    right: 20px;
    background: var(--accent);
    color: white;
    width: 60px;
    height: 60px;
    border-radius: 50%;
    display: flex;
    align-items: center;
    justify-content: center;
    font-size: 1.8rem;
    text-decoration: none;
    box-shadow: 0 4px 12px rgba(0, 0, 0, 0.3);
    transition: all 0.3s ease;
    z-index: 1000;
    cursor: pointer;
}

.feedback-button:hover {
    transform: scale(1.1) rotate(5deg);
    box-shadow: 0 6px 20px rgba(59, 130, 246, 0.5);
}

.feedback-button:active {
    transform: scale(0.95);
}

@media (max-width: 768px) {
    .feedback-button {
        bottom: 15px;
        right: 15px;
        width: 50px;
        height: 50px;
        font-size: 1.5rem;
    }
}

/* Print styles for better printing */
@media print {
    body {
        background: white !important;
        color: black !important;
    }
    
    .theme-selector,
    .feedback-button,
    .share-btn {
        display: none !important;
    }
    
    .content-area {
        box-shadow: none !important;
        border: 1px solid #ccc !important;
    }
    
    .problem-display {
        font-size: 2rem !important;
        color: black !important;
    }
    
    .lesson-content h2,
    .lesson-content h3 {
        color: black !important;
    }
    
    .lesson-content p,
    .lesson-content li {
        color: #333 !important;
    }
}