:root {
    /* Color Palette */
    --color-bg: #121212; /* Matte black / dark charcoal */
    --color-surface: #1e1e1e;
    --color-surface-hover: #2a2a2a;
    --color-primary: #7a00e6; /* Deep vibrant royal purple */
    --color-primary-hover: #9b30ff;
    --color-text-main: #b0b8c1; /* Light slate gray */
    --color-text-heading: #ffffff;
    --color-border: #333333;
    
    /* Typography */
    --font-sans: 'Inter', sans-serif;
    --font-math: 'Noto Serif', serif; /* Standard serif for Sigma symbol */
    
    /* Spacing & Layout */
    --spacing-sm: 0.5rem;
    --spacing-md: 1rem;
    --spacing-lg: 2rem;
    --spacing-xl: 2.5rem;
    
    /* Transitions */
    --transition-fast: 0.2s ease;
}

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

html {
    scroll-behavior: smooth;
}

body {
    font-family: var(--font-sans);
    background-color: var(--color-bg);
    color: var(--color-text-main);
    line-height: 1.6;
    overflow-x: hidden;
}

h1, h2, h3, h4 {
    color: var(--color-text-heading);
    font-weight: 700;
    line-height: 1.2;
}

a {
    text-decoration: none;
    color: inherit;
    transition: color var(--transition-fast);
}

/* Typography & Symbols */
.sigma {
    font-family: var(--font-math);
    font-size: 1.5rem;
    font-weight: normal;
    font-style: italic;
    color: var(--color-primary);
    margin-right: 0.25rem;
}

.sigma-large {
    font-family: var(--font-math);
    font-size: 22rem;
    font-weight: normal;
    font-style: italic;
    color: rgba(122, 0, 230, 0.1); /* Subtle large background sigma */
    background: linear-gradient(135deg, var(--color-primary) 0%, rgba(122, 0, 230, 0.05) 100%);
    -webkit-background-clip: text;
    -webkit-text-fill-color: transparent;
    line-height: 1;
    user-select: none;
    animation: float 6s ease-in-out infinite;
}

@keyframes float {
    0% { transform: translateY(0px); }
    50% { transform: translateY(-20px); }
    100% { transform: translateY(0px); }
}

/* Navbar */
.navbar {
    display: flex;
    justify-content: space-between;
    align-items: center;
    padding: var(--spacing-md) 5%;
    background-color: rgba(18, 18, 18, 0.85);
    backdrop-filter: blur(12px);
    position: fixed;
    top: 0;
    width: 100%;
    z-index: 100;
    border-bottom: 1px solid var(--color-border);
}

.logo {
    display: flex;
    align-items: center;
    font-size: 1.25rem;
    font-weight: 600;
    color: var(--color-text-heading);
    letter-spacing: -0.5px;
}

.navbar nav {
    display: flex;
    gap: var(--spacing-lg);
    align-items: center;
}

.navbar nav a:not(.btn-secondary):not(.btn-primary):hover {
    color: var(--color-text-heading);
}

/* Buttons */
.btn-primary {
    display: inline-block;
    background-color: var(--color-primary);
    color: #ffffff;
    padding: 0.875rem 1.75rem;
    border-radius: 4px;
    font-weight: 600;
    transition: all var(--transition-fast);
    border: 1px solid var(--color-primary);
    box-shadow: 0 2px 10px rgba(122, 0, 230, 0.2);
}

.btn-primary:hover {
    background-color: var(--color-primary-hover);
    border-color: var(--color-primary-hover);
    box-shadow: 0 6px 20px rgba(122, 0, 230, 0.5);
    transform: translateY(-2px);
    color: #ffffff;
}

.btn-secondary {
    display: inline-block;
    color: var(--color-text-heading);
    font-weight: 600;
}

.btn-secondary:hover {
    color: var(--color-primary-hover);
}

.btn-outline {
    display: inline-block;
    background-color: transparent;
    color: var(--color-text-heading);
    padding: 0.875rem 1.75rem;
    border-radius: 4px;
    font-weight: 600;
    transition: all var(--transition-fast);
    border: 1px solid var(--color-border);
}

.btn-outline:hover {
    border-color: var(--color-text-main);
    background-color: rgba(255, 255, 255, 0.05);
}

/* Hero Section */
.hero {
    min-height: 70vh;
    display: flex;
    align-items: center;
    padding: var(--spacing-xl) 5%;
    padding-top: 6rem; /* Account for fixed nav */
    position: relative;
}

.hero-content {
    flex: 1;
    max-width: 600px;
    z-index: 2;
}

.hero-visual {
    flex: 1;
    display: flex;
    justify-content: center;
    align-items: center;
    z-index: 1;
}

.badge {
    display: inline-block;
    padding: 0.3rem 0.85rem;
    background-color: rgba(122, 0, 230, 0.15);
    color: var(--color-primary-hover);
    border-radius: 20px;
    font-size: 0.875rem;
    font-weight: 600;
    margin-bottom: var(--spacing-md);
    border: 1px solid rgba(122, 0, 230, 0.3);
    letter-spacing: 0.5px;
    text-transform: uppercase;
}

.hero h1 {
    font-size: 3.5rem;
    margin-bottom: var(--spacing-md);
    letter-spacing: -1px;
}

.highlight {
    color: var(--color-primary-hover);
}

.hero p {
    font-size: 1.125rem;
    margin-bottom: var(--spacing-lg);
    max-width: 500px;
}

.cta-group {
    display: flex;
    gap: var(--spacing-md);
    flex-wrap: wrap;
}

/* Features Section */
.features {
    padding: var(--spacing-lg) 5%;
    background-color: var(--color-surface);
    border-top: 1px solid var(--color-border);
}

.features h2 {
    text-align: center;
    font-size: 2.5rem;
    margin-bottom: var(--spacing-xl);
}

.feature-grid {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(300px, 1fr));
    gap: var(--spacing-lg);
    max-width: 1200px;
    margin: 0 auto;
}

.card {
    background-color: var(--color-bg);
    padding: var(--spacing-lg);
    border-radius: 12px;
    border: 1px solid var(--color-border);
    transition: transform var(--transition-fast), border-color var(--transition-fast), box-shadow var(--transition-fast);
}

.card:hover {
    transform: translateY(-8px);
    border-color: var(--color-primary);
    box-shadow: 0 10px 30px rgba(0, 0, 0, 0.5);
}

.card-icon {
    width: 56px;
    height: 56px;
    background-color: rgba(122, 0, 230, 0.1);
    color: var(--color-primary-hover);
    display: flex;
    justify-content: center;
    align-items: center;
    border-radius: 50%;
    font-size: 1.5rem;
    font-weight: 700;
    margin-bottom: var(--spacing-md);
    border: 1px solid rgba(122, 0, 230, 0.2);
}

.card h3 {
    font-size: 1.25rem;
    margin-bottom: var(--spacing-sm);
}

/* Footer */
footer {
    padding: var(--spacing-lg) 5%;
    border-top: 1px solid var(--color-border);
    background-color: var(--color-bg);
}

.footer-content {
    display: flex;
    justify-content: space-between;
    align-items: center;
    max-width: 1200px;
    margin: 0 auto;
    flex-wrap: wrap;
    gap: var(--spacing-md);
}

.footer-content p {
    font-size: 0.875rem;
}

/* Reviews Page Styles */
.reviews-section {
    padding: var(--spacing-xl) 5%;
    padding-top: 8rem;
    max-width: 1000px;
    margin: 0 auto;
}

.reviews-header {
    margin-bottom: var(--spacing-lg);
    display: flex;
    justify-content: space-between;
    align-items: center;
    flex-wrap: wrap;
    gap: var(--spacing-md);
}

.filters {
    display: flex;
    gap: var(--spacing-sm);
    flex-wrap: wrap;
}

.filter-btn {
    background-color: var(--color-surface);
    color: var(--color-text-main);
    border: 1px solid var(--color-border);
    padding: 0.5rem 1.2rem;
    border-radius: 20px;
    cursor: pointer;
    font-size: 0.875rem;
    font-weight: 600;
    transition: all var(--transition-fast);
}

.filter-btn.active, .filter-btn:hover {
    background-color: rgba(122, 0, 230, 0.15);
    color: var(--color-text-heading);
    border-color: var(--color-primary);
}

.sort-select {
    background-color: var(--color-surface);
    color: var(--color-text-heading);
    border: 1px solid var(--color-border);
    padding: 0.6rem 1rem;
    border-radius: 4px;
    font-family: inherit;
    font-size: 0.875rem;
    cursor: pointer;
    outline: none;
}

.sort-select:focus {
    border-color: var(--color-primary);
}

.reviews-grid {
    display: flex;
    flex-direction: column;
    gap: var(--spacing-lg);
}

.review-card {
    background-color: var(--color-surface);
    border: 1px solid var(--color-border);
    border-radius: 12px;
    padding: var(--spacing-lg);
    transition: transform var(--transition-fast), box-shadow var(--transition-fast), border-color var(--transition-fast);
    animation: fadeIn 0.4s ease-out forwards;
}

.review-card:hover {
    transform: translateY(-4px);
    box-shadow: 0 8px 24px rgba(0, 0, 0, 0.3);
    border-color: var(--color-primary);
}

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

.review-header {
    display: flex;
    justify-content: space-between;
    align-items: flex-start;
    margin-bottom: var(--spacing-md);
    flex-wrap: wrap;
    gap: var(--spacing-sm);
}

.review-title {
    font-size: 1.25rem;
    color: var(--color-text-heading);
    margin-bottom: 0.25rem;
}

.review-meta {
    font-size: 0.875rem;
    color: var(--color-text-main);
}

.review-meta strong {
    color: var(--color-text-heading);
}

.review-stars {
    color: #f59e0b; /* Golden yellow for stars */
    font-size: 1.1rem;
    letter-spacing: 2px;
}

.review-text {
    margin-bottom: var(--spacing-md);
    line-height: 1.6;
}

.tutor-response {
    margin-top: var(--spacing-md);
    padding: var(--spacing-md) var(--spacing-lg);
    background-color: rgba(122, 0, 230, 0.05);
    border-left: 3px solid var(--color-primary);
    border-radius: 0 8px 8px 0;
}

.tutor-response-label {
    font-weight: 700;
    color: var(--color-primary-hover);
    margin-bottom: 0.5rem;
    font-size: 0.875rem;
    text-transform: uppercase;
    letter-spacing: 0.5px;
}

.tutor-response-text {
    font-size: 0.95rem;
    line-height: 1.5;
    color: var(--color-text-main);
}

/* Responsive Design */
@media (max-width: 768px) {
    /* Stack the fixed header: brand on top, nav links wrapping below */
    .navbar {
        flex-direction: column;
        align-items: center;
        gap: var(--spacing-sm);
        padding: var(--spacing-sm) var(--spacing-md);
    }

    .navbar nav {
        width: 100%;
        flex-wrap: wrap;
        justify-content: center;
        gap: var(--spacing-sm) var(--spacing-md);
    }

    .navbar nav a {
        padding: 0.25rem var(--spacing-sm);
        text-align: center;
    }

    .navbar nav .btn-primary {
        flex-basis: 100%;
        max-width: 20rem;
        margin: 0 auto;
        padding: 0.75rem 1.5rem;
    }

    .hero {
        flex-direction: column;
        text-align: center;
        padding-top: 11rem; /* Clear the taller stacked fixed nav */
    }

    .reviews-section {
        padding-top: 11rem; /* Clear the taller stacked fixed nav */
    }
    
    .hero-content {
        display: flex;
        flex-direction: column;
        align-items: center;
    }
    
    .cta-group {
        justify-content: center;
    }
    
    .hero-visual {
        margin-top: var(--spacing-xl);
    }
    
    .hero h1 {
        font-size: 2.5rem;
    }

    .sigma-large {
        font-size: 16rem;
    }
}