:root {
    /* Enhanced color palette */
    --bg-primary: #0d1117;
    --bg-secondary: #161b22;
    --bg-tertiary: #21262d;
    --bg-hover: #2d333b;
    --text-primary: #f0f6fc;
    --text-secondary: #8b949e;
    --accent: #238636;
    --accent-hover: #2ea043;
    --danger: #da3633;
    --danger-hover: #f85149;
    --success: #238636;
    --warning: #9e6a03;
    --border: #30363d;
    --radius: 6px;
    --shadow: 0 4px 6px -1px rgba(0, 0, 0, 0.1), 0 2px 4px -1px rgba(0, 0, 0, 0.06);
    --transition: all 0.2s ease-in-out;
}

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

body { 
    background: var(--bg-primary);
    color: var(--text-primary);
    font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, Arial, sans-serif;
    line-height: 1.6;
    min-height: 100vh;
}

.container {
    max-width: 1200px;
    margin: 0 auto;
    padding: 30px;
}

h1, h2, h3 {
    margin-bottom: 25px;
    font-weight: 500;
    color: var(--text-primary);
}

h1 { font-size: 32px; }
h2 { font-size: 24px; }
h3 { font-size: 20px; }

/* Cards */
.card {
    background: var(--bg-secondary);
    border-radius: var(--radius);
    padding: 25px;
    margin-bottom: 20px;
    border: 1px solid var(--border);
}

/* Forms */
.form-group {
    margin-bottom: 20px;
}

.form-group label {
    display: block;
    margin-bottom: 8px;
    color: var(--text-secondary);
    font-weight: 500;
}

input, textarea, select {
    width: 100%;
    padding: 12px;
    margin: 8px 0;
    border: 1px solid var(--border);
    background: var(--bg-tertiary);
    color: var(--text-primary);
    border-radius: var(--radius);
    font-size: 14px;
}

input:focus, textarea:focus, select:focus {
    outline: none;
    border-color: var(--accent);
}

/* Buttons */
.btn {
    background: var(--accent);
    color: white;
    padding: 12px 24px;
    border: none;
    border-radius: var(--radius);
    cursor: pointer;
    font-size: 14px;
    font-weight: 500;
    text-decoration: none;
    display: inline-block;
    transition: all 0.2s;
}

.btn:hover {
    background: var(--accent-hover);
    transform: translateY(-1px);
}

.btn-large {
    font-size: 16px;
    padding: 15px 30px;
}

.btn-danger {
    background: var(--danger);
}

.btn-danger:hover {
    background: var(--danger-hover);
}

/* Tables */
table {
    width: 100%;
    border-collapse: separate;
    border-spacing: 0;
    margin-bottom: 25px;
    background: var(--bg-secondary);
    border-radius: var(--radius);
    overflow: hidden;
}

th, td {
    padding: 15px;
    text-align: left;
    border-bottom: 1px solid var(--border);
}

th {
    background: var(--bg-tertiary);
    font-weight: 500;
    color: var(--text-secondary);
}

tr:last-child td {
    border-bottom: none;
}

tr:hover {
    background: var(--bg-tertiary);
}

/* Messages */
.msg {
    background: var(--bg-tertiary);
    color: var(--accent);
    padding: 15px;
    margin-bottom: 25px;
    border-radius: var(--radius);
    font-weight: 500;
    text-align: center;
}

.msg.error {
    color: var(--danger);
}

/* Challenges */
.challenge-grid {
    display: grid;
    grid-template-columns: repeat(auto-fill, minmax(300px, 1fr));
    gap: 20px;
    margin: 20px 0;
}

.challenge-card {
    background: var(--bg-secondary);
    border-radius: var(--radius);
    padding: 20px;
    border: 1px solid var(--border);
}

.challenge-card h3 {
    margin-bottom: 15px;
}

.challenge-card .points {
    color: var(--accent);
    font-weight: 500;
    margin-bottom: 15px;
}

.challenge-card.solved {
    border-color: var(--accent);
}

/* Scoreboard */
.score-card {
    display: flex;
    align-items: center;
    justify-content: space-between;
    background: var(--bg-secondary);
    padding: 15px 20px;
    margin-bottom: 10px;
    border-radius: var(--radius);
    border: 1px solid var(--border);
}

.score-card .rank {
    font-size: 24px;
    font-weight: bold;
    color: var(--accent);
    margin-right: 20px;
}

.score-card .points {
    font-weight: 500;
}

/* Navigation */
.nav {
    background: var(--bg-secondary);
    padding: 15px 0;
    border-bottom: 1px solid var(--border);
}

.nav .container {
    display: flex;
    justify-content: space-between;
    align-items: center;
    padding: 0 30px;
}

.nav a {
    color: var(--text-primary);
    text-decoration: none;
    padding: 8px 15px;
    border-radius: var(--radius);
}

.nav a:hover {
    background: var(--bg-tertiary);
}

/* Responsive */
@media (max-width: 768px) {
    .container {
        padding: 15px;
    }
    
    .challenge-grid {
        grid-template-columns: 1fr;
    }
    
    table {
        display: block;
        overflow-x: auto;
    }
    
    .nav .container {
        flex-direction: column;
        gap: 10px;
    }
}