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

:root {
    --bg-gradient-start: #f0f4f8;
    --bg-gradient-end: #e2e8f0;
    --header-bg: rgba(255, 255, 255, 0.98);
    --text-primary: #1a202c;
    --text-secondary: #4a5568;
    --text-muted: #718096;
    --nav-link-bg: #e2e8f0;
    --nav-link-hover: #cbd5e0;
    --active-bg: #1e3a8a;
    --active-color: white;
    --card-bg: #ffffff;
    --card-shadow: 0 10px 25px rgba(0,0,0,0.1);
    --info-bg: #e0f2fe;
    --info-border: #0284c7;
    --code-bg: #f1f5f9;
    --code-color: #dc2626;
    --btn-bg: #1e3a8a;
    --btn-hover: #1e40af;
    --btn-secondary-bg: white;
    --btn-secondary-color: #1e3a8a;
    --btn-secondary-border: #1e3a8a;
    --error-bg: #fee2e2;
    --error-border: #dc2626;
    --error-color: #991b1b;
    --border-color: #e2e8f0;
}

body.dark {
    --bg-gradient-start: #0f172a;
    --bg-gradient-end: #1e293b;
    --header-bg: rgba(15, 23, 42, 0.98);
    --text-primary: #f1f5f9;
    --text-secondary: #cbd5e0;
    --text-muted: #94a3b8;
    --nav-link-bg: #334155;
    --nav-link-hover: #475569;
    --active-bg: #3b82f6;
    --active-color: white;
    --card-bg: #1e293b;
    --card-shadow: 0 10px 25px rgba(0,0,0,0.3);
    --info-bg: #1e3a8a;
    --info-border: #60a5fa;
    --code-bg: #0f172a;
    --code-color: #f87171;
    --btn-bg: #3b82f6;
    --btn-hover: #2563eb;
    --btn-secondary-bg: #1e293b;
    --btn-secondary-color: #3b82f6;
    --btn-secondary-border: #3b82f6;
    --error-bg: #7f1d1d;
    --error-border: #ef4444;
    --error-color: #fecaca;
    --border-color: #334155;
}

body {
    font-family: system-ui, -apple-system, 'Segoe UI', Roboto, sans-serif;
    background: linear-gradient(135deg, var(--bg-gradient-start) 0%, var(--bg-gradient-end) 100%);
    color: var(--text-primary);
    min-height: 100vh;
    transition: background 0.3s ease, color 0.3s ease;
}

header {
    background: var(--header-bg);
    backdrop-filter: blur(10px);
    box-shadow: 0 2px 10px rgba(0,0,0,0.1);
    padding: 1rem 2rem;
    position: sticky;
    top: 0;
    z-index: 10;
    transition: background 0.3s ease;
}

nav {
    max-width: 1200px;
    margin: 0 auto;
    display: flex;
    gap: 1rem;
    justify-content: center;
    flex-wrap: wrap;
    align-items: center;
}

nav a {
    color: var(--text-secondary);
    text-decoration: none;
    font-weight: 500;
    padding: 0.5rem 1.5rem;
    border-radius: 0.5rem;
    transition: all 0.3s ease;
    cursor: pointer;
}

nav a:hover {
    background: var(--nav-link-hover);
    transform: translateY(-2px);
}

nav a.active {
    background: var(--active-bg);
    color: var(--active-color);
    box-shadow: 0 4px 6px rgba(0,0,0,0.1);
}

.theme-switch {
    margin-left: 1rem;
    position: relative;
    display: inline-block;
    width: 60px;
    height: 34px;
}

.theme-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: 0.4s;
    border-radius: 34px;
}

.slider:before {
    position: absolute;
    content: "☀️";
    display: flex;
    align-items: center;
    justify-content: center;
    height: 26px;
    width: 26px;
    left: 4px;
    bottom: 4px;
    background-color: white;
    transition: 0.4s;
    border-radius: 50%;
    font-size: 14px;
}

input:checked + .slider {
    background-color: #1e3a8a;
}

input:checked + .slider:before {
    transform: translateX(26px);
    content: "🌙";
}

main {
    max-width: 1200px;
    margin: 2rem auto;
    padding: 0 2rem;
}

#content {
    min-height: 400px;
}

.fade-in {
    animation: fadeInScale 0.5s cubic-bezier(0.4, 0, 0.2, 1);
}

@keyframes fadeInScale {
    from {
        opacity: 0;
        transform: scale(0.98) translateY(10px);
    }
    to {
        opacity: 1;
        transform: scale(1) translateY(0);
    }
}

.loading {
    text-align: center;
    padding: 3rem;
    font-size: 1.2rem;
    color: var(--text-secondary);
}

.loader {
    width: 50px;
    height: 50px;
    border: 3px solid rgba(30, 58, 138, 0.3);
    border-radius: 50%;
    border-top-color: var(--active-bg);
    animation: spin 1s ease-in-out infinite;
    margin: 0 auto 1rem;
}

body.dark .loader {
    border: 3px solid rgba(59, 130, 246, 0.3);
    border-top-color: var(--active-bg);
}

@keyframes spin {
    to { transform: rotate(360deg); }
}

.card {
    background: var(--card-bg);
    border-radius: 1rem;
    padding: 2rem;
    box-shadow: var(--card-shadow);
    margin-bottom: 1.5rem;
    transition: background 0.3s ease, box-shadow 0.3s ease;
}

.card h1 {
    font-size: 2rem;
    margin-bottom: 1rem;
    color: var(--text-primary);
}

.card h2 {
    font-size: 1.5rem;
    margin: 1rem 0 0.5rem;
    color: var(--text-primary);
}

.card p {
    color: var(--text-secondary);
    line-height: 1.6;
    margin-bottom: 0.5rem;
}

.card ul, .card ol {
    color: var(--text-secondary);
    margin-left: 1.5rem;
    margin-top: 0.5rem;
}

.card li {
    margin: 0.5rem 0;
}

.card hr {
    margin: 1rem 0;
    border-color: var(--border-color);
}

.info {
    background: var(--info-bg);
    border-left: 4px solid var(--info-border);
    padding: 1rem;
    margin-top: 1rem;
    border-radius: 0.5rem;
    color: var(--text-primary);
}

.error {
    background: var(--error-bg);
    border-left: 4px solid var(--error-border);
    padding: 1rem;
    border-radius: 0.5rem;
    color: var(--error-color);
}

code {
    background: var(--code-bg);
    padding: 0.2rem 0.4rem;
    border-radius: 0.3rem;
    font-family: monospace;
    color: var(--code-color);
}

button {
    background: var(--btn-bg);
    color: white;
    border: none;
    padding: 0.6rem 1.2rem;
    border-radius: 0.5rem;
    font-weight: 500;
    cursor: pointer;
    transition: all 0.2s;
}

button:hover {
    background: var(--btn-hover);
    transform: translateY(-2px);
    box-shadow: 0 4px 6px rgba(0,0,0,0.1);
}

.counter {
    font-size: 3rem;
    font-weight: bold;
    margin: 1rem 0;
    color: var(--active-bg);
}

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

.nav-buttons button {
    background: var(--btn-secondary-bg);
    color: var(--btn-secondary-color);
    border: 2px solid var(--btn-secondary-border);
}

.nav-buttons button:hover {
    background: var(--btn-secondary-border);
    color: white;
    transform: translateY(-2px);
}

.retry-btn {
    margin-top: 1rem;
    background: var(--btn-bg);
    color: white;
}