:root {
    /* Light Mode - Trust & Calm */
    --primary-color: #0056b3; /* Deep Trust Blue */
    --secondary-color: #00a8cc; /* Calm Teal */
    --accent-color: #e3f2fd; /* Light Blue Background Accent */
    --text-color: #212529; /* Dark Charcoal */
    --bg-color: #f8f9fa; /* Off-White / Cream */
    --card-bg: #ffffff;
    --border-color: #dee2e6;
    --link-color: #0056b3;
    --link-hover: #003d82;
}

[data-bs-theme="dark"] {
    /* Dark Mode - Modern & Futuristic */
    --primary-color: #63b3ed; /* Bright Sky Blue */
    --secondary-color: #9f7aea; /* Soft Purple Accent */
    --accent-color: #172a45; /* Darker Navy Accent */
    --text-color: #e2e8f0; /* Off-White / Light Gray */
    --bg-color: #0a192f; /* Deep Navy Blue */
    --card-bg: #112240; /* Slightly Lighter Navy */
    --border-color: #233554;
    --link-color: #63b3ed;
    --link-hover: #90cdf4;
}

body {
    font-family: 'Roboto', sans-serif;
    display: flex;
    flex-direction: column;
    min-height: 100vh;
    background-color: var(--bg-color);
    color: var(--text-color);
    transition: background-color 0.3s ease, color 0.3s ease;
}

main {
    flex: 1;
}

/* Typography & Links */
h1, h2, h3, h4, h5, h6 {
    color: var(--text-color);
    font-weight: 700;
}

a {
    color: var(--link-color);
    text-decoration: none;
    transition: color 0.2s;
}

a:hover {
    color: var(--link-hover);
    text-decoration: underline;
}

/* Buttons */
.btn-primary {
    background-color: var(--primary-color);
    border-color: var(--primary-color);
    color: #ffffff; /* Always white on primary */
}

[data-bs-theme="dark"] .btn-primary {
    color: #0a192f; /* Dark text on bright blue in dark mode for contrast */
    font-weight: 600;
}

.btn-primary:hover {
    background-color: var(--link-hover);
    border-color: var(--link-hover);
}

.btn-outline-primary {
    color: var(--primary-color);
    border-color: var(--primary-color);
}

.btn-outline-primary:hover {
    background-color: var(--primary-color);
    color: #fff;
    border-color: var(--primary-color);
}

/* Cards */
.card {
    background-color: var(--card-bg);
    border: 1px solid var(--border-color);
    border-radius: 8px;
}

.card-title {
    color: var(--primary-color); /* Pop of color */
}

[data-bs-theme="dark"] .card-title {
    color: var(--secondary-color); /* Purple accent in dark mode */
}

/* Navbar */
.navbar {
    background-color: var(--card-bg) !important;
    border-bottom: 1px solid var(--border-color);
}

.navbar-nav .nav-link {
    font-weight: 500;
    transition: all 0.2s ease;
    padding-bottom: 0.25rem;
    margin: 0 0.5rem;
}

.navbar-nav .nav-link:hover {
    color: var(--primary-color) !important;
    transform: translateY(-1px);
}

.navbar-nav .nav-link.active {
    color: var(--primary-color) !important;
    border-bottom: 2px solid var(--primary-color);
}

[data-bs-theme="dark"] .navbar-brand {
    color: var(--primary-color) !important;
}

/* Hero Section */
.hero {
    position: relative;
    background-size: cover;
    background-position: center;
}

.hero::before {
    content: '';
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    /* Gradient overlay for text readability */
    background: linear-gradient(135deg, rgba(10, 25, 47, 0.85) 0%, rgba(0, 86, 179, 0.6) 100%);
}

.hero .container {
    position: relative;
    z-index: 2;
}

.hero h1, .hero p {
    color: #ffffff !important;
}

.text-shadow {
    text-shadow: 0 2px 4px rgba(0,0,0,0.5);
}

.backdrop-blur {
    backdrop-filter: blur(10px);
    background-color: rgba(255, 255, 255, 0.1);
    border: 1px solid rgba(255, 255, 255, 0.2);
}

/* Utility */
.bg-light {
    background-color: var(--bg-color) !important;
}

.bg-body-tertiary {
    background-color: var(--card-bg) !important;
}

.bg-card-bg {
    background-color: var(--card-bg) !important;
}

.article-content img {
    max-width: 100%;
    height: auto;
    border-radius: 8px;
    margin: 2rem 0;
    box-shadow: 0 4px 12px rgba(0,0,0,0.15);
}

/* Form Inputs */
.form-control, .form-select {
    background-color: var(--bg-color);
    border-color: var(--border-color);
    color: var(--text-color);
}

[data-bs-theme="dark"] .form-control:focus {
    background-color: var(--card-bg);
    color: var(--text-color);
    border-color: var(--primary-color);
    box-shadow: 0 0 0 0.25rem rgba(99, 179, 237, 0.25);
}

/* List Groups (Filters) */
.list-group-item {
    background-color: var(--card-bg);
    border-color: var(--border-color);
    color: var(--text-color);
}

[data-bs-theme="dark"] .list-group-item {
    color: var(--text-color);
}

[data-bs-theme="dark"] .list-group-item:hover,
[data-bs-theme="dark"] .list-group-item.active {
    background-color: var(--accent-color);
    border-color: var(--primary-color);
    color: var(--primary-color);
    font-weight: 600;
}

/* Book Cards */
.book-card {
    transition: transform 0.2s;
    background-color: var(--bg-color); /* Slight contrast against white container */
}

[data-bs-theme="dark"] .book-card {
    background-color: var(--card-bg);
    border: 1px solid var(--border-color) !important;
}

.book-card:hover {
    transform: translateY(-5px);
}

.book-icon-wrapper {
    background-color: var(--accent-color);
    width: 80px;
    height: 80px;
    border-radius: 50%;
    display: flex;
    align-items: center;
    justify-content: center;
}