:root {
    --primary: #4a90e2;
    --accent: #e94e77;
    --success: #28a745;
    --dark: #1e1e1e;
}

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

body {
    font-family: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif;
    background: linear-gradient(135deg, #f5f7fa 0%, #c3cfe2 100%);
    color: #333;
    line-height: 1.6;
}

header {
    background: linear-gradient(135deg, var(--primary), #2c6e9c);
    color: white;
    padding: 2rem 0;
    text-align: center;
    box-shadow: 0 4px 15px rgba(0,0,0,0.1);
}

.logo {
    font-size: 2.8rem;
    font-weight: bold;
    margin-bottom: 0.5rem;
}

nav {
    background: white;
    padding: 1rem;
    text-align: center;
    box-shadow: 0 2px 8px rgba(0,0,0,0.1);
}

nav a {
    color: var(--primary);
    text-decoration: none;
    font-weight: bold;
}

.main-content {
    max-width: 1100px;
    margin: 2rem auto;
    padding: 0 20px;
}

.content-area {
    background: white;
    padding: 2.5rem;
    border-radius: 16px;
    box-shadow: 0 6px 25px rgba(0,0,0,0.1);
}

.code-block {
    background: var(--dark);
    color: #d4d4d4;
    padding: 1.5rem;
    border-radius: 10px;
    font-family: 'Consolas', 'Courier New', monospace;
    overflow-x: auto;
    position: relative;
    margin: 1.5rem 0;
}

.run-btn {
    position: absolute;
    top: 12px;
    right: 12px;
    background: var(--success);
    color: white;
    border: none;
    padding: 8px 16px;
    border-radius: 6px;
    cursor: pointer;
    font-size: 0.95rem;
}

.output {
    background: #f0f8f0;
    border: 1px solid #a8e6a8;
    padding: 1.2rem;
    border-radius: 8px;
    margin-top: 1rem;
    font-family: monospace;
}

.exercise {
    background: #fff9e6;
    border-left: 6px solid #f4c542;
    padding: 1.8rem;
    margin: 2rem 0;
    border-radius: 10px;
}

.solution {
    display: none;
    background: #e6f7e6;
    padding: 1.5rem;
    border-radius: 8px;
    margin-top: 1rem;
    border: 1px solid #a8e6a8;
}

.btn {
    background: var(--accent);
    color: white;
    border: none;
    padding: 12px 24px;
    border-radius: 8px;
    cursor: pointer;
    margin: 8px 5px 8px 0;
    font-weight: 500;
}

.btn:hover {
    background: #d43a66;
    transform: translateY(-2px);
}

h1, h2, h3 {
    color: var(--primary);
    margin: 1.5rem 0 1rem 0;
}